Tools that implement these rules shall respect the following syntax to explicitly suppress a rule:. Some rules are hard to check mechanically, but they all meet the minimal criteria that an expert programmer can spot many violations without too much trouble. We hope that "mechanical" tools will improve with time to approximate what such an expert programmer notices.
Also, we assume that the rules will be refined over time to make them more precise and checkable. A rule is aimed at being simple, rather than carefully phrased to mention every alternative and special case. Such information is found in the Alternative paragraphs and the Discussion sections. If you don't understand a rule or disagree with it, please visit its Discussion.
If you feel that a discussion is missing or incomplete, enter an Issue explaining your concerns and possibly a corresponding PR. This is not a language manual. It is meant to be helpful, rather than complete, fully accurate on technical details, or a guide to existing code.
Recommended information sources can be found in the references. Each section e. The main section abbreviations are also used in rule numbers e. Philosophical rules are generally not mechanically checkable. However, individual rules reflecting these philosophical themes are. Compilers don't read comments or design documents and neither do many programmers consistently. What is expressed in code has defined semantics and can in principle be checked by compilers and other tools. The first declaration of month is explicit about returning a Month and about not modifying the state of the Date object.
The second version leaves the reader guessing and opens more possibilities for uncaught bugs. A well-designed library expresses intent what is to be done, rather than just how something is being done far better than direct use of language features. Any programmer should know the basics of the foundation libraries of the project being worked on, and use them appropriately.
Any programmer using these guidelines should know the guidelines support library , and use it appropriately. A better approach is to be explicit about the meaning of the double new speed or delta on old speed? We could have accepted a plain unit-less double as a delta, but that would have been error-prone. If we wanted both absolute speed and deltas, we would have defined a Delta type. There are environments where extensions are necessary, e.
In such cases, localize the use of necessary extensions and control their use with non-core Coding Guidelines. If possible, build interfaces that encapsulate the extensions so they can be turned off or compiled away on systems that do not support those extensions. Extensions often do not have rigorously defined semantics. Even extensions that are common and implemented by multiple compilers may have slightly different behaviors and edge case behavior as a direct result of not having a rigorous standard definition.
With sufficient use of any such extension, expected portability will be impacted. Avoid dependence on undefined behavior e. In such cases, control their dis use with an extension of these Coding Guidelines customized to the specific environment. Unless the intent of some code is stated e. The intent of "just" looping over the elements of v is not expressed here. The implementation detail of an index is exposed so that it might be misused , and i outlives the scope of the loop, which may or may not be intended.
The reader cannot know from just this section of code. Now, there is no explicit mention of the iteration mechanism, and the loop operates on a reference to const elements so that accidental modification cannot happen. If modification is desired, say so:. For more details about for-statements, see ES. Sometimes better still, use a named algorithm. The last variant makes it clear that we are not interested in the order in which the elements of v are handled.
Ideally, a program would be completely statically compile-time type safe. Unfortunately, that is not possible. Problem areas:. These areas are sources of serious problems e. We try to provide alternative techniques. We can ban, restrain, or detect the individual problem categories separately, as required and feasible for individual programs. Always suggest an alternative.
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For example:. Code clarity and performance. You don't need to write error handlers for errors caught at compile time. Alternative formulation : Don't postpone to run time what can be done well at compile time. Ideally, we catch all errors that are not errors in the programmer's logic at either compile time or run time. It is impossible to catch all errors at compile time and often not affordable to catch all remaining errors at run time.
However, we should endeavor to write programs that in principle can be checked, given sufficient resources analysis programs, run-time checks, machine resources, time. Here, a crucial bit of information the number of elements has been so thoroughly "obscured" that static analysis is probably rendered infeasible and dynamic checking can be very difficult when f is part of an ABI so that we cannot "instrument" that pointer. We could embed helpful information into the free store, but that requires global changes to a system and maybe to the compiler.
The Divided Self
What we have here is a design that makes error detection very hard. Passing the number of elements as an argument is better and far more common than just passing the pointer and relying on some unstated convention for knowing or discovering the number of elements. However as shown , a simple typo can introduce a serious error. The connection between the two arguments of f2 is conventional, rather than explicit.
Also, it is implicit that f2 is supposed to delete its argument or did the caller make a second mistake? The standard library resource management pointers fail to pass the size when they point to an object:. This design carries the number of elements along as an integral part of an object, so that errors are unlikely and dynamic run-time checking is always feasible, if not always affordable. Here we made a small error in use1 that will lead to corrupted data or a crash. The pointer, count -style interface leaves increment1 with no realistic way of defending itself against out-of-range errors.
If we could check subscripts for out of range access, then the error would not be discovered until p was accessed. We could check earlier and improve the code:. If all we had was a typo so that we meant to use n as the bound, the code could be further simplified eliminating the possibility of an error :. The date is validated twice by the Date constructor and passed as a character string unstructured data. Excess checking can be costly. There are cases where checking early is dumb because you may not ever need the value, or may only need part of the value that is more easily checked than the whole.
Similarly, don't add validity checks that change the asymptotic behavior of your interface e. Even a slow growth in resources will, over time, exhaust the availability of those resources. This is particularly important for long-running programs, but is an essential piece of responsible programming behavior. A leak is colloquially "anything that isn't cleaned up. This rule should not be taken as requiring that allocations within long-lived objects must be returned during program shutdown.
For example, relying on system guaranteed cleanup such as file closing and memory deallocation upon process shutdown can simplify code. However, relying on abstractions that implicitly clean up can be as simple, and often safer. Enforcing the lifetime safety profile eliminates leaks. When combined with resource safety provided by RAII , it eliminates the need for "garbage collection" by generating no garbage.
Combine this with enforcement of the type and bounds profiles and you get complete type- and resource-safety, guaranteed by tools. Time and space that you spend well to achieve a goal e. Yes, this is a caricature, but we have seen every individual mistake in production code, and worse. Note that the layout of X guarantees that at least 6 bytes and most likely more are wasted.
The spurious definition of copy operations disables move semantics so that the return operation is slow please note that the Return Value Optimization, RVO, is not guaranteed here. The use of new and delete for buf is redundant; if we really needed a local string, we should use a local string. There are several more performance bugs and gratuitous complication. This is actually an example from production code. This expression will be evaluated on every iteration of the loop, which means that strlen must walk through string every loop to discover its length.
While the string contents are changing, it's assumed that toLower will not affect the length of the string, so it's better to cache the length outside the loop and not incur that cost each iteration. An individual example of waste is rarely significant, and where it is significant, it is typically easily eliminated by an expert. However, waste spread liberally across a code base can easily be significant and experts are not always as available as we would like. After that, we can look at waste related to algorithms and requirements, but that is beyond the scope of these guidelines.
Many more specific rules aim at the overall goals of simplicity and elimination of gratuitous waste. It is easier to reason about constants than about variables. Something immutable cannot change unexpectedly. Sometimes immutability enables better optimization. You can't have a data race on a constant.
Messy code is more likely to hide bugs and harder to write. A good interface is easier and safer to use. Messy, low-level code breeds more such code. This is low-level, verbose, and error-prone. For example, we "forgot" to test for memory exhaustion. Instead, we could use vector :. The standards library and the GSL are examples of this philosophy.
For example, instead of messing with the arrays, unions, cast, tricky lifetime issues, gsl::owner , etc. Similarly, we can and should design and implement more specialized libraries, rather than leaving the users often ourselves with the challenge of repeatedly getting low-level code well. This is a variant of the subset of superset principle that underlies these guidelines. There are many things that are done better "by machine".
Computers don't tire or get bored by repetitive tasks. We typically have better things to do than repeatedly do routine tasks. There are many other kinds of tools, such as source code repositories, build tools, etc. Be careful not to become dependent on over-elaborate or over-specialized tool chains.
Those can make your otherwise portable code non-portable. Using a well-designed, well-documented, and well-supported library saves time and effort; its quality and documentation are likely to be greater than what you could do if the majority of your time must be spent on an implementation. The cost time, effort, money, etc. A widely used library is more likely to be kept up-to-date and ported to new systems than an individual application. So, if a suitable library exists for your application domain, use it. Unless you are an expert in sorting algorithms and have plenty of time, this is more likely to be correct and to run faster than anything you write for a specific application.
You need a reason not to use the standard library or whatever foundational libraries your application uses rather than a reason to use it. If no well-designed, well-documented, and well-supported library exists for an important domain, maybe you should design and implement it, and then use it. An interface is a contract between two parts of a program. Precisely stating what is expected of a supplier of a service and a user of that service is essential. Having good easy-to-understand, encouraging efficient use, not error-prone, supporting testing, etc.
Controlling the behavior of a function through a global namespace scope variable a call mode is implicit and potentially confusing. It will not be obvious to a caller that the meaning of two calls of round 7. Sometimes we control the details of a set of operations by an environment variable, e. The use of a non-local control is potentially confusing, but controls only implementation details of otherwise fixed semantics. Alternative formulation : Avoid passing information across an interface through non-local or implicit state.
Note that non- const member functions pass information to other member functions through their object's state. Alternative formulation : An interface should be a function or a set of functions. Functions can be template functions and sets of functions can be classes or class templates. Non- const global variables hide dependencies and make the dependencies subject to unpredictable changes. Alternative : If you use global more generally namespace scope data to avoid copying, consider passing the data as an object by reference to const. Another solution is to define the data as the state of some object and the operations as member functions.
Warning : Beware of data races: If one thread can access nonlocal data or data passed by reference while another thread executes the callee, we can have a data race. Every pointer or reference to mutable data is a potential data race. The rule is "avoid", not "don't use. If you don't want a global object to change, declare it const or constexpr. You can use the simplest "singleton" so simple that it is often not considered a singleton to get initialization on first use, if any:.
This is one of the most effective solutions to problems related to initialization order. In a multi-threaded environment, the initialization of the static object does not introduce a race condition unless you carelessly access a shared object from within its constructor. Note that the initialization of a local static does not imply a race condition. However, if the destruction of X involves an operation that needs to be synchronized we must use a less simple solution. Now someone must delete that object in some suitably thread-safe way.
That's error-prone, so we don't use that technique unless. If you, as many do, define a singleton as a class for which only one object is created, functions like myX are not singletons, and this useful technique is not an exception to the no-singleton rule. Types are the simplest and best documentation, improve legibility due to their well-defined meaning, and are checked at compile time. Also, precisely typed code is often optimized better. Callers are unsure what types are allowed and if the data may be mutated as const is not specified.
That is error-prone and verbose. Consider using a variant or a pointer to base instead. For generic code these T s can be general or concept constrained template parameters. It is clear that the caller is describing a rectangle, but it is unclear what parts they relate to. Also, an int can carry arbitrary forms of information, including values of many units, so we must guess about the meaning of the four int s. Most likely, the first two are an x , y coordinate pair, but what are the last two?
Obviously, we cannot catch all errors through the static type system e. The parameter types and their values do not communicate what settings are being specified or what those values mean. For the case of a set of boolean values consider using a flags enum; a pattern that expresses a set of boolean values. Here x must be nonnegative.
The type system cannot easily and naturally express that, so we must use other means. For now, we place it in the definition function body. Prefer a formal specification of requirements, such as Expects p ;. Most member functions have as a precondition that some class invariant holds. That invariant is established by a constructor and must be reestablished upon exit by every member function called from outside the class.
We don't need to mention it for each member function. Preconditions can be stated in many ways, including comments, if -statements, and assert. This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and may have the wrong semantics do you always want to abort in debug mode and check nothing in productions runs? Preconditions should be part of the interface rather than part of the implementation, but we don't yet have the language facilities to do that.
Once language support becomes available e. No, using unsigned is not a good way to sidestep the problem of ensuring that a value is nonnegative. Not enforceable Finding the variety of ways preconditions can be asserted is not feasible. Warning about those that can be easily identified assert has questionable value in the absence of a language facility. Here, we incautiously left out the precondition specification, so it is not explicit that height and width must be positive.
Overflow can happen. Consider using:. There was no postcondition stating that the buffer should be cleared and the optimizer eliminated the apparently redundant memset call:. Postconditions are often informally stated in a comment that states the purpose of a function; Ensures can be used to make this more systematic, visible, and checkable.
Postconditions are especially important when they relate to something that is not directly reflected in a returned result, such as a state of a data structure used.
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Consider a function that manipulates a Record , using a mutex to avoid race conditions:. Here, we "forgot" to state that the mutex should be released, so we don't know if the failure to ensure release of the mutex was a bug or a feature. Stating the postcondition would have made it clear:. Better still, use RAII to ensure that the postcondition "the lock must be released" is enforced in code:.
Only postconditions related to the users can be stated in the interface. Not enforceable This is a philosophical guideline that is infeasible to check directly in the general case. Domain specific checkers like lock-holding checkers exist for many toolchains. Postconditions can be stated in many ways, including comments, if -statements, and assert.
This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and may have the wrong semantics. Alternative : Postconditions of the form "this resource must be released" are best expressed by RAII. Ideally, that Ensures should be part of the interface, but that's not easily done. Not enforceable Finding the variety of ways postconditions can be asserted is not feasible. Make the interface precisely specified and compile-time checkable in the not so distant future.
Concepts are supported in GCC 6. See also : Generic programming and concepts. Not yet enforceable A language facility is under specification. When the language facility is available, warn if any non-variadic template parameter is not constrained by a concept in its declaration or mentioned in a requires clause. It should not be possible to ignore an error because that could leave the system or a computation in an undefined or unexpected state.
This is a major source of errors. An error means that the function cannot achieve its advertised purpose including establishing postconditions. Calling code that ignores an error could lead to wrong results or undefined systems state. For example, not being able to connect to a remote server is not by itself an error: the server can refuse a connection for all kinds of reasons, so the natural thing is to return a result that the caller should always check.
However, if failing to make a connection is considered an error, then a failure should throw an exception. Many traditional interface functions e. You don't have a good alternative to using such, so calling these does not violate the rule. If you can't use exceptions e. This style unfortunately leads to uninitialized variables. See also : I. If there is any doubt whether the caller or the callee owns an object, leaks or premature destruction will occur. Who deletes the returned X? The problem would be harder to spot if compute returned a reference.
Consider returning the result by value use move semantics if the result is large :. However, that is less elegant and often less efficient than returning the object itself, so use smart pointers only if reference semantics are needed. Alternative : Sometimes older code can't be modified because of ABI compatibility requirements or lack of resources. In that case, mark owning pointers using owner from the guidelines support library :. This tells analysis tools that res is an owner.
That is, its value must be delete d or transferred to another owner, as is done here by the return. Every object passed as a raw pointer or iterator is assumed to be owned by the caller, so that its lifetime is handled by the caller. Viewed another way: ownership transferring APIs are relatively rare compared to pointer-passing APIs, so the default is "no ownership transfer. See also : Argument passing , use of smart pointer arguments , and value return.
To help avoid dereferencing nullptr errors. To improve performance by avoiding redundant checks for nullptr. By stating the intent in source, implementers and tools can provide better diagnostics, such as finding some classes of errors through static analysis, and perform optimizations, such as removing branches and null tests. The assumption that the pointer to char pointed to a C-style string a zero-terminated string of characters was still implicit, and a potential source of confusion and errors. Also, a plain pointer to array must rely on some convention to allow the callee to determine the size.
What if there are fewer than n elements in the array pointed to by q? Then, we overwrite some probably unrelated memory. What if there are fewer than n elements in the array pointed to by p? Then, we read some probably unrelated memory. Either is undefined behavior and a potentially very nasty bug. Passing 10 as the n argument may be a mistake: the most common convention is to assume [0:n but that is nowhere stated. Worse is that the call of draw compiled at all: there was an implicit conversion from array to pointer array decay and then another implicit conversion from Circle to Shape.
There is no way that draw can safely iterate through that array: it has no way of knowing the size of the elements. Alternative : Use a support class that ensures that the number of elements is correct and prevents dangerous implicit conversions. This draw2 passes the same amount of information to draw , but makes the fact that it is supposed to be a range of Circle s explicit. Use zstring and czstring to represent C-style, zero-terminated strings. Since x and y are in different translation units the order of calls to f and g is undefined; one will access an uninitialized const. This shows that the order-of-initialization problem for global namespace scope objects is not limited to global variables.
Order of initialization problems become particularly difficult to handle in concurrent code. It is usually best to avoid global namespace scope objects altogether. Having many arguments opens opportunities for confusion. Passing lots of arguments is often costly compared to alternatives.
Missing an abstraction. There is an abstraction missing, so that a compound value is being passed as individual elements instead of as a single object that enforces an invariant. This not only expands the parameter list, but it leads to errors because the component values are no longer protected by an enforced invariant. Violating "one function, one responsibility. Note that this is because of problem 1 above -- missing abstraction. Instead of passing a range abstraction , STL passed iterator pairs unencapsulated component values.
Here, we have four template arguments and six function arguments. This doesn't reduce the total complexity, but it reduces the surface complexity presented to many users. To really reduce the number of arguments, we need to bundle the arguments into higher-level abstractions:. Grouping arguments into "bundles" is a general technique to reduce the number of arguments and to increase the opportunities for checking. Alternatively, we could use concepts as defined by the ISO TS to define the notion of three types that must be usable for merging:.
Here, using an abstraction has safety and robustness benefits, and naturally also reduces the number of parameters. How many parameters are too many? Try to use fewer than four 4 parameters. There are functions that are best expressed with four individual parameters, but not many.
Alternative : Use better abstraction: Group arguments into meaningful objects and pass the objects by value or by reference. Alternative : Use default arguments or overloads to allow the most common forms of calls to be done with fewer arguments. It is easy to reverse the "to" and "from" arguments. Define a struct as the parameter type and name the fields for those parameters accordingly:. This tends to make invocations of this clear to future readers, as the parameters are often filled in by name at the call site.
This will force every derived class to compute a center -- even if that's non-trivial and the center is never used. Similarly, not every Shape has a Color , and many Shape s are best represented without an outline defined as a sequence of Point s. Abstract classes were invented to discourage users from writing such classes:.
Different compilers implement different binary layouts for classes, exception handling, function names, and other implementation details. That may require recompilation after an upgrade to a new compiler version. Because private data members participate in class layout and private member functions participate in overload resolution, changes to those implementation details require recompilation of all users of a class that uses them.
A non-polymorphic interface class holding a pointer to implementation Pimpl can isolate the users of a class from changes in its implementation at the cost of an indirection. See GOTW and cppreference for the trade-offs and additional implementation details associated with this idiom. To keep code simple and safe. Sometimes, ugly, unsafe, or error-prone techniques are necessary for logical or performance reasons. If so, keep them local, rather than "infecting" interfaces so that larger groups of programmers have to be aware of the subtleties. Implementation complexity should, if at all possible, not leak through interfaces into user code.
Consider a program that, depending on some form of input e. We might write. This violated the rule against uninitialized variables , the rule against ignoring ownership , and the rule against magic constants. In particular, someone has to remember to somewhere write. The common, most frequent, and safest examples can be handled statically, so we don't want to add cost and complexity to those. But we must also cope with the uncommon, less-safe, and necessarily more expensive cases.
Such examples are discussed in [Str15]. Now, the dynamic nature of istream ownership has been encapsulated. Presumably, a bit of checking for potential errors would be added in real code. A function specifies an action or a computation that takes the system from one consistent state to the next. It is the fundamental building block of programs. It should be possible to name a function meaningfully, to specify the requirements of its argument, and clearly state the relationship between the arguments and the result. An implementation is not a specification. Try to think about what a function does as well as about how it does it.
Functions are the most critical part in most interfaces, so see the interface rules. A function definition is a function declaration that also specifies the function's implementation, the function body. Factoring out common code makes code more readable, more likely to be reused, and limit errors from complex code. If something is a well-specified action, separate it out from its surrounding code and give it a name.
It reads, it writes to a fixed ostream , it writes error messages to a fixed ostream , it handles only int s. There is nothing to reuse, logically separate operations are intermingled and local variables are in scope after the end of their logical use. For a tiny example, this looks OK, but if the input operation, the output operation, and the error handling had been more complicated the tangled mess could become hard to understand. If you write a non-trivial lambda that potentially can be used in more than one place, give it a name by assigning it to a usually non-local variable.
Naming that lambda breaks up the expression into its logical parts and provides a strong hint to the meaning of the lambda. Loop bodies, including lambdas used as loop bodies, rarely need to be named. However, large loop bodies e. The rule Keep functions short and simple implies "Keep loop bodies short. This is a monolith that is tied to a specific input and will never find another different use.
Instead, break functions up into suitable logical parts and parameterize:. Large functions are hard to read, more likely to contain complex code, and more likely to have variables in larger than minimal scopes. Functions with complex control structures are more likely to be long and more likely to hide logical errors.
This is too complex. How would you know if all possible alternatives have been correctly handled? Yes, it breaks other rules also. Break large functions up into smaller cohesive and named functions. Small simple functions are easily inlined where the cost of a function call is significant. Don't try to make all functions constexpr. Most computation is best done at run time. Any API that may eventually depend on high-level run-time configuration or business logic should not be made constexpr. Such customization can not be evaluated by the compiler, and any constexpr functions that depended upon that API would have to be refactored or drop constexpr.
Impossible and unnecessary. The compiler gives an error if a non- constexpr function is called where a constant is required. Some optimizers are good at inlining without hints from the programmer, but don't rely on it. Over the last 40 years or so, we have been promised compilers that can inline better than humans without hints from humans. We are still waiting. Specifying inline encourages the compiler to do a better job. Do not put an inline function in what is meant to be a stable interface unless you are certain that it will not change. An inline function is part of the ABI.
Template functions incl. Flag inline functions that are more than three statements and could have been declared out of line such as class member functions. If an exception is not supposed to be thrown, the program cannot be assumed to cope with the error and should be terminated as soon as possible. Declaring a function noexcept helps optimizers by reducing the number of alternative execution paths. It also speeds up the exit after failure. Put noexcept on every function written completely in C or in any other language without exceptions.
If collect runs out of memory, the program crashes. Unless the program is crafted to survive memory exhaustion, that may be just the right thing to do; terminate may generate suitable error log information but after memory runs out it is hard to do anything clever. You must be aware of the execution environment that your code is running when deciding whether to tag a function noexcept , especially because of the issue of throwing and allocation. However, most programs and execution environments cannot meaningfully handle a failure to allocate, and aborting the program is the cleanest and simplest response to an allocation failure in those cases.
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If you know that your application code cannot respond to an allocation failure, it may be appropriate to add noexcept even on functions that allocate. Put another way: In most programs, most functions can throw e. Destructors, swap functions, move operations, and default constructors should never throw. See also C. Passing a smart pointer transfers or shares ownership and should only be used when ownership semantics are intended see R. Passing by smart pointer restricts the use of a function to callers that use smart pointers. Passing a shared smart pointer e.
We can catch dangling pointers statically, so we don't need to rely on resource management to avoid violations from dangling pointers. Pure functions are easier to reason about, sometimes easier to optimize and even parallelize , and sometimes can be memoized. Using "unusual and clever" techniques causes surprises, slows understanding by other programmers, and encourages bugs.
Both let the caller know that a function will not modify the argument, and both allow initialization by rvalues. What is "cheap to copy" depends on the machine architecture, but two or three words doubles, pointers, references are usually best passed by value. When copying is cheap, nothing beats the simplicity and safety of copying, and for small objects up to two or three words it is also faster than passing by reference because it does not require an extra indirection to access from the function. For advanced uses only , where you really need to optimize for rvalues passed to "input-only" parameters:.
Assuming that Matrix has move operations possibly by keeping its elements in a std::vector :. A reference may be assumed to refer to a valid object language rule. There is no legitimate "null reference. That can in itself be a problem and a source of errors:. Here, the writer of g is supplying a buffer for f to fill, but f simply replaces it at a somewhat higher cost than a simple copy of the characters.
A bad logic error can happen if the writer of g incorrectly assumes the size of the buffer. That could be dangerous. Passing by value does generate one extra cheap move operation, but prefer simplicity and clarity first. If the object is to be passed onward to other code and not directly used by this function, we want to make this function agnostic to the argument const -ness and rvalue-ness.
This includes large objects like standard containers that use implicit move operations for performance and to avoid explicit memory management. If you have multiple values to return, use a tuple or similar multi-member type. A struct of many individually cheap-to-move elements may be in aggregate expensive to move.
It is not recommended to return a const value. Such older advice is now obsolete; it does not add value, and it interferes with move semantics.
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The argument for adding const to a return value is that it prevents very rare accidental access to a temporary. The argument against is prevents very frequent use of move semantics. A return value is self-documenting as an "output-only" value. Prefer using a named struct where there are semantics to the returned value. Otherwise, a nameless tuple is useful in generic code.
Sometimes, we need to pass an object to a function to manipulate its state. Explicitly passing an in-out parameter back out again as a return value is often not necessary. Here, both s and cin are used as in-out parameters. We pass cin by non- const reference to be able to manipulate its state. We pass s to avoid repeated allocations. By reusing s passed by reference , we allocate new memory only when we need to expand s 's capacity. This technique is sometimes called the "caller-allocated out" pattern and is particularly useful for types, such as string and vector , that needs to do free store allocations.
For a truly strict reading of this rule F. However, we prefer to be explicit, rather than subtle. The overly-generic pair and tuple should be used only when the value returned represents independent entities rather than an abstraction. This makes it hard to understand what the code does and is supposed to do.
It complicates checking and tool support. When I call length p should I check if p is nullptr first? Should the implementation of length check if p is nullptr? Typically, they are implicit and their correct use is very hard to ensure. A span represents a range of elements, but how do we manipulate elements of that range? Passing a span object as an argument is exactly as efficient as passing a pair of pointer arguments or passing a pointer and an integer count. Complex Warn where accesses to pointer parameters are bounded by other parameters that are integral types and suggest they could use span instead.
C-style strings are ubiquitous. They are defined by convention: zero-terminated arrays of characters. We must distinguish C-style strings from a pointer to a single character or an old-fashioned pointer to an array of characters. When I call length s should I check if s is nullptr first? See also : C. You need to pass a pointer rather than an object if what you are transferring is an object from a class hierarchy that is to be used through an interface base class.
Simple Warn if a function returns a locally allocated raw pointer. That is, the last owner deletes the object. Have a single object own the shared object e. Sometimes having nullptr as an alternative to indicated "no object" is useful, but if it is not, a reference is notationally simpler and might yield better code. That error is very uncommon. That's what pointers are good for.
If it isn't the nullptr , the pointer returned by find indicates a Node holding s. Importantly, that does not imply a transfer of ownership of the pointed-to object to the caller. Positions can also be transferred by iterators, indices, and references. A reference is often a superior alternative to a pointer if there is no need to use nullptr or if the object referred to should not change. Do not return a pointer to something that is not in the caller's scope; see F. To avoid the crashes and data corruption that can result from the use of such a dangling pointer.
This applies only to non- static local variables. All static variables are as their name indicates statically allocated, so that pointers to them cannot dangle. Here I managed to read the location abandoned by the call of f. The pointer stored in glob could be used much later and cause trouble in unpredictable ways.
Similar examples can be constructed "leaking" a pointer from an inner scope to an outer one; such examples are handled equivalently to leaks of pointers out of a function. A slightly different variant of the problem is placing pointers in a container that outlives the objects pointed to.
See also : Another way of getting dangling pointers is pointer invalidation. See also : The return of a reference must not imply transfer of ownership: discussion of dangling pointer prevention and discussion of ownership. It's asking to return a reference to a destroyed temporary object. A returned rvalue reference goes out of scope at the end of the full expression to which it is returned:.
This kind of use is a frequent source of bugs, often incorrectly reported as a compiler bug. An implementer of a function should avoid setting such traps for users. The lifetime safety profile will when completely implemented catch such problems. Returning an rvalue reference is fine when the reference to the temporary is being passed "downward" to a callee; then, the temporary is guaranteed to outlive the function call see F.
However, it's not fine when passing such a reference "upward" to a larger caller scope. It's a language rule, but violated through "language extensions" so often that it is worth mentioning. Declaring main the one global main of a program void limits portability. This ensures consistency with standard-library types and follows the principle of "do as the ints do.
This should be enforced by tooling by checking the return type and return value of any assignment operator. With guaranteed copy elision, it is now almost always a pessimization to expressly use std::move in a return statement. Functions can't capture local variables or be declared at local scope; if you need those things, prefer a lambda where possible, and a handwritten function object where not. On the other hand, lambdas and function objects don't overload; if you need to overload, prefer a function the workarounds to make lambdas overload are ornate.
If either will work, prefer writing a function; use the simplest tool necessary. Generic lambdas offer a concise way to write function templates and so can be useful even when a normal function template would do equally well with a little more syntax. This advantage will probably disappear in the future once all functions gain the ability to have Concept parameters. Default arguments simply provide alternative interfaces to a single implementation. There is no guarantee that a set of overloaded functions all implement the same semantics.
The use of default arguments can avoid code replication. There is a choice between using default argument and overloading when the alternatives are from a set of arguments of the same types. There is not a choice when a set of functions are used to do a semantically equivalent operation to a set of types. For efficiency and correctness, you nearly always want to capture by reference when using the lambda locally.
This includes when writing or calling parallel algorithms that are local because they join before returning. The correctness consideration is that many calls want to perform side effects on the original object at the call site see example below. Passing by value prevents this. Unfortunately, there is no simple way to capture by reference to const to get the efficiency for a local call but also prevent side effects.
Here, a large object a network message is passed to an iterative algorithm, and is it not efficient or correct to copy the message which may not be copyable :. This is a simple three-stage parallel pipeline. Each stage object encapsulates a worker thread and a queue, has a process function to enqueue work, and in its destructor automatically blocks waiting for the queue to empty before ending the thread. Flag a lambda that captures by reference, but is used other than locally within the function scope or passed to a function by reference.
Note: This rule is an approximation, but does flag passing by pointer as those are more likely to be stored by the callee, writing to a heap location accessed via a parameter, returning the lambda, etc. The Lifetime rules will also provide general rules that flag escaping pointers and references including via lambdas.
Pointers and references to locals shouldn't outlive their scope. Lambdas that capture by reference are just another place to store a reference to a local object, and shouldn't do so if they or a copy outlive the scope. It's confusing. If you meant to do that, write this explicitly. For now, just be explicit. Passing to varargs assumes the correct type will be read.
This is fragile because it cannot generally be enforced to be safe in the language and so relies on programmer discipline to get it right. Declaring a A class is a user-defined type, for which a programmer can define the representation, operations, and interfaces. Class hierarchies are used to organize related classes into hierarchical structures. Ease of comprehension. If data is related for fundamental reasons , that fact should be reflected in code. From a language perspective class and struct differ only in the default visibility of their members.
The use of class alerts the programmer to the need for an invariant. This is a useful convention. An invariant is a logical condition for the members of an object that a constructor must establish for the public member functions to assume. After the invariant is established typically by a constructor every member function can be called for the object.
An invariant can be stated informally e. If a class has any private data, a user cannot completely initialize an object without the use of a constructor. Hence, the class definer will provide a constructor and must specify its meaning. This effectively means the definer need to define an invariant. Look for struct s with all data private and class es with public members.
An explicit distinction between interface and implementation improves readability and simplifies maintenance. For example, we can now change the representation of a Date without affecting its users recompilation is likely, though. Using a class in this way to represent the distinction between interface and implementation is of course not the only way. For example, we can use a set of declarations of freestanding functions in a namespace, an abstract base class, or a template function with concepts to represent an interface.
The most important issue is to explicitly distinguish between an interface and its implementation "details. Less coupling than with member functions, fewer functions that can cause trouble by modifying object state, reduces the number of functions that needs to be modified after a change in representation. The "helper functions" have no need for direct access to the representation of a Date.
The language requires virtual functions to be members, and not all virtual functions directly access data. In particular, members of an abstract class rarely do. Note multi-methods.
A helper function is a function usually supplied by the writer of a class that does not need direct access to the representation of the class, yet is seen as part of the useful interface to the class. Placing them in the same namespace as the class makes their relationship to the class obvious and allows them to be found by argument dependent lookup. Mixing a type definition and the definition of another entity in the same declaration is confusing and unnecessary. The private data is hidden far from the public data. The data is split in different parts of the class declaration.
Different parts of the data have different access. All of this decreases readability and complicates maintenance. Flag classes declared with struct if there is a private or protected member. Information hiding. Minimize the chance of unintended access. This simplifies maintenance. In a large code base, we cannot easily find which code does what to the members of pair. This may be exactly what we want, but if we want to enforce a relation among members, we need to make them private and enforce that relation invariant through constructors and member functions.
For public and protected data, that's usually the case. A class can provide two interfaces to its users. One for derived classes protected and one for general users public. For example, a derived class might be allowed to skip a run-time check because it has already guaranteed correctness:. Prefer the order public members before protected members before private members see.
One ideal for a class is to be a regular type. That means roughly "behaves like an int. A value of regular type can be copied and the result of a copy is an independent object with the same value as the original. Concrete classes without assignment and equality can be defined, but they are and should be rare. Concrete types are also often referred to as value types to distinguish them from types used as part of a hierarchy.
A concrete type is fundamentally simpler than a hierarchy: easier to design, easier to implement, easier to use, easier to reason about, smaller, and faster. You need a reason use cases for using a hierarchy. If a class can be part of a hierarchy, we in real code if not necessarily in small examples must manipulate its objects through pointers or references. About this product. Stock photo. Pre-owned: lowest price The lowest-priced item that has been used or worn previously. Shows some signs of wear, and may have some markings on the inside. Shipped to over one million happy customers.
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