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These are implemented as macros in C and as functions in C: Classification macro / functions fpclassify Classify floating-point value (macro/function ) isfinite Is finite value (macro ) isinf Is infinity (macro/function ) isnan Is Not-A-Number (macro/function ) isnormal Is normal (macro/function ) signbit Sign bit (macro/function ). This reference explains the C programming language as implemented in the Microsoft C compiler. The organization is based on The Annotated C Reference Manual by Margaret Ellis and Bjarne Stroustrup and on the ANSI/ISO C International Standard (ISO/IEC FDIS 14882). Microsoft-specific implementations of C language features are included.

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User-Defined Type Conversions (C) A conversion produces a new value of some type from a value of a different type. Standard conversions are built into the C language and support its built-in types, and you can create user-defined conversions to perform conversions to, from, or between user-defined types.
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A function is a block of code that performs some operation. A function can optionally define input parameters that enable callers to pass arguments into the function. A function can optionally return a value as output. Functions are useful for encapsulating common operations in a single reusable block, ideally with a name that clearly describes what the function does. The following function accepts two integers from a caller and returns their sum; a and b are parameters of type int.

The function can be invoked, or called, from any number of places in the program. The values that are passed to the function are the arguments, whose types must be compatible with the parameter types in the function definition.

There is no practical limit to function length, but good design aims for functions that perform a single well-defined task. Complex algorithms should be broken up into easy-to-understand simpler functions whenever possible.

Functions that are defined at class scope are called member functions. In C++, unlike other languages, a function can also be defined at namespace scope (including the implicit global namespace). Such functions are called free functions or non-member functions; they are used extensively in the Standard Library.

Functions may be overloaded, which means different versions of a function may share the same name if they differ by the number and/or type of formal parameters. For more information, see Function Overloading.

Parts of a function declaration

A minimal function declaration consists of the return type, function name, and parameter list (which may be empty), along with optional keywords that provide additional instructions to the compiler. The following example is a function declaration:

A function definition consists of a declaration, plus the body, which is all the code between the curly braces:

A function declaration followed by a semicolon may appear in multiple places in a program. It must appear prior to any calls to that function in each translation unit. The function definition must appear only once in the program, according to the One Definition Rule (ODR).

The required parts of a function declaration are:

The return type, which specifies the type of the value that the function returns, or void if no value is returned. In C++11, auto is a valid return type that instructs the compiler to infer the type from the return statement. In C++14, decltype(auto) is also allowed. For more information, see Type Deduction in Return Types below.
The function name, which must begin with a letter or underscore and cannot contain spaces. In general, leading underscores in the Standard Library function names indicate private member functions, or non-member functions that are not intended for use by your code.
The parameter list, a brace delimited, comma-separated set of zero or more parameters that specify the type and optionally a local name by which the values may be accessed inside the function body.

Optional parts of a function declaration are:

constexpr, which indicates that the return value of the function is a constant value can be computed at compile time.
Its linkage specification, extern or static.
For more information, see Translation units and linkage.
inline, which instructs the compiler to replace every call to the function with the function code itself. inlining can help performance in scenarios where a function executes quickly and is invoked repeatedly in a performance-critical section of code.
For more information, see Inline Functions.
A noexcept expression, which specifies whether or not the function can throw an exception. In the following example, the function does not throw an exception if the is_pod expression evaluates to true.
For more information, see noexcept.
(Member functions only) The cv-qualifiers, which specify whether the function is const or volatile.
(Member functions only) virtual, override, or final. virtual specifies that a function can be overridden in a derived class. override means that a function in a derived class is overriding a virtual function. final means a function cannot be overridden in any further derived class. For more information, see Virtual Functions.
(member functions only) static applied to a member function means that the function is not associated with any object instances of the class.
(Non-static member functions only) The ref-qualifier, which specifies to the compiler which overload of a function to choose when the implicit object parameter (*this) is an rvalue reference vs. an lvalue reference. For more information, see Function Overloading.

The following figure shows the parts of a function definition. The shaded area is the function body.

Parts of a function definition

Function definitions

A function definition consists of the declaration and the function body, enclosed in curly braces, which contains variable declarations, statements and expressions. The following example shows a complete function definition:

Variables declared inside the body are called local variables or locals. They go out of scope when the function exits; therefore, a function should never return a reference to a local!

const and constexpr functions

You can declare a member function as const to specify that the function is not allowed to change the values of any data members in the class. By declaring a member function as const, you help the compiler to enforce const-correctness. If someone mistakenly tries to modify the object by using a function declared as const, a compiler error is raised. For more information, see const.

Declare a function as constexpr when the value it produces can possibly be determined at compile time. A constexpr function generally executes faster than a regular function. For more information, see constexpr.

Function Templates

A function template is similar to a class template; it generates concrete functions based on the template arguments. In many cases, the template is able to infer the type arguments and therefore it isn't necessary to explicitly specify them.

For more information, see Function Templates

Function parameters and arguments

A function has a comma-separated parameter list of zero or more types, each of which has a name by which it can be accessed inside the function body. A function template may specify additional type or value parameters. The caller passes arguments, which are concrete values whose types are compatible with the parameter list.

By default, arguments are passed to the function by value, which means the function receives a copy of the object being passed. For large objects, making a copy can be expensive and is not always necessary. To cause arguments to be passed by reference (specifically lvalue reference), add a reference quantifier to the parameter:

When a function modifies an argument that is passed by reference, it modifies the original object, not a local copy. To prevent a function from modifying such an argument, qualify the parameter as const&:

C++ 11: To explicitly handle arguments that are passed by rvalue-reference or lvalue-reference, use a double-ampersand on the parameter to indicate a universal reference:

A function declared with the single keyword void in the parameter declaration list takes no arguments, as long as the keyword void is the first and only member of the argument declaration list. Arguments of type void elsewhere in the list produce errors. For example:

Note that, while it is illegal to specify a void argument except as outlined here, types derived from type void (such as pointers to void and arrays of void) can appear anywhere the argument declaration list.

Default Arguments

The last parameter or parameters in a function signature may be assigned a default argument, which means that the caller may leave out the argument when calling the function unless they want to specify some other value.

For more information, see Default Arguments.

Function return types

A function may not return another function, or a built-in array; however it can return pointers to these types, or a lambda, which produces a function object. Except for these cases, a function may return a value of any type that is in scope, or it may return no value, in which case the return type is void.

Trailing return types

An 'ordinary' return type is located on the left side of the function signature. A trailing return type is located on the right most side of the signature and is preceded by the -> operator. Trailing return types are especially useful in function templates when the type of the return value depends on template parameters.

When auto is used in conjunction with a trailing return type, it just serves as a placeholder for whatever the decltype expression produces, and does not itself perform type deduction.

Function local variables

A variable that is declared inside a function body is called a local variable or simply a local. Non-static locals are only visible inside the function body and, if they are declared on the stack go out of scope when the function exits. When you construct a local variable and return it by value, the compiler can usually perform the named return value optimization to avoid unnecessary copy operations. If you return a local variable by reference, the compiler will issue a warning because any attempt by the caller to use that reference will occur after the local has been destroyed.

In C++ a local variable may be declared as static. The variable is only visible inside the function body, but a single copy of the variable exists for all instances of the function. Local static objects are destroyed during termination specified by atexit. If a static object was not constructed because the program's flow of control bypassed its declaration, no attempt is made to destroy that object.

Type deduction in return types (C++14)

In C++14, you can use auto to instruct the compiler to infer the return type from the function body without having to provide a trailing return type. Note that auto always deduces to a return-by-value. Use auto&& to instruct the compiler to deduce a reference.

In this example, auto will be deduced as a non-const value copy of the sum of lhs and rhs.

Note that auto does not preserve the const-ness of the type it deduces. For forwarding functions whose return value needs to preserve the const-ness or ref-ness of its arguments, you can use the decltype(auto) keyword, which uses the decltype type inference rules and preserves all the type information. decltype(auto) may be used as an ordinary return value on the left side, or as a trailing return value.

The following example (based on code from N3493), shows decltype(auto) being used to enable perfect forwarding of function arguments in a return type that isn't known until the template is instantiated.

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Returning multiple values from a function

There are various ways to return more than one value from a function:

Encapsulate the values in a named class or struct object. Requires the class or struct definition to be visible to the caller:
Return a std::tuple or std::pair object:
Visual Studio 2017 version 15.3 and later (available with /std:c++17): Use structured bindings. The advantage of structured bindings is that the variables that store the return values are initialized at the same time they are declared, which in some cases can be significantly more efficient. In this statement --auto[x, y, z] = f();-- the brackets introduce and initialize names that are in scope for the entire function block.
In addition to using the return value itself, you can 'return' values by defining any number of parameters to use pass-by-reference so that the function can modify or initialize the values of objects that the caller provides. For more information, see Reference-Type Function Arguments.

Function pointers

C++ supports function pointers in the same manner as the C language. However a more type-safe alternative is usually to use a function object.

It is recommended that typedef be used to declare an alias for the function pointer type if declaring a function that returns a function pointer type. For example

If this is not done, the proper syntax for the function declaration may be deduced from the declarator syntax for the function pointer by replacing the identifier (fp in the above example) with the functions name and argument list, as follows:

The preceding declaration is equivalent to the declaration using typedef above.

The explicit keyword and problems with implicit conversion

By default when you create a user-defined conversion, the compiler can use it to perform implicit conversions. Sometimes this is what you want, but other times the simple rules that guide the compiler in making implicit conversions can lead it to accept code that you don't want it to.

One well-known example of an implicit conversion that can cause problems is the conversion to bool. There are many reasons that you might want to create a class type that can be used in a Boolean context—for example, so that it can be used to control an if statement or loop—but when the compiler performs a user-defined conversion to a built-in type, the compiler is allowed to apply an additional standard conversion afterwards. The intent of this additional standard conversion is to allow for things like promotion from short to int, but it also opens the door for less-obvious conversions—for example, from bool to int, which allows your class type to be used in integer contexts you never intended. This particular problem is known as the Safe Bool Problem. This kind of problem is where the explicit keyword can help.

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The explicit keyword tells the compiler that the specified conversion can't be used to perform implicit conversions. If you wanted the syntactic convenience of implicit conversions before the explicit keyword was introduced, you had to either accept the unintended consequences that implicit conversion sometimes created or use less-convenient, named conversion functions as a workaround. Now, by using the explicit keyword, you can create convenient conversions that can only be used to perform explicit casts or direct initialization, and that won't lead to the kind of problems exemplified by the Safe Bool Problem.

The explicit keyword can be applied to conversion constructors since C++98, and to conversion functions since C++11. The following sections contain more information about how to use the explicit keyword.

Conversion constructors

Conversion constructors define conversions from user-defined or built-in types to a user-defined type. The following example demonstrates a conversion constructor that converts from the built-in type double to a user-defined type Money.

Notice that the first call to the function display_balance, which takes an argument of type Money, doesn't require a conversion because its argument is the correct type. However, on the second call to display_balance, a conversion is needed because the type of the argument, a double with a value of 49.95, is not what the function expects. The function can't use this value directly, but because there's a conversion from the type of the argument—double—to the type of the matching parameter—Money—a temporary value of type Money is constructed from the argument and used to complete the function call. In the third call to display_balance, notice that the argument is not a double, but is instead a float with a value of 9.99—and yet the function call can still be completed because the compiler can perform a standard conversion—in this case, from float to double—and then perform the user-defined conversion from double to Money to complete the necessary conversion.

Declaring conversion constructors

The following rules apply to declaring a conversion constructor:

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The target type of the conversion is the user-defined type that's being constructed.
Conversion constructors typically take exactly one argument, which is of the source type. However, a conversion constructor can specify additional parameters if each additional parameter has a default value. The source type remains the type of the first parameter.
Conversion constructors, like all constructors, do not specify a return type. Specifying a return type in the declaration is an error.
Conversion constructors can be explicit.

Explicit conversion constructors

By declaring a conversion constructor to be explicit, it can only be used to perform direct initialization of an object or to perform an explicit cast. This prevents functions that accept an argument of the class type from also implicitly accepting arguments of the conversion constructor's source type, and prevents the class type from being copy-initialized from a value of the source type. The following example demonstrates how to define an explicit conversion constructor, and the effect it has on what code is well-formed.

In this example, notice that you can still use the explicit conversion constructor to perform direct initialization of payable. If instead you were to copy-initialize Money payable = 79.99;, it would be an error. The first call to display_balance is unaffected because the argument is the correct type. The second call to display_balance is an error, because the conversion constructor can't be used to perform implicit conversions. The third call to display_balance is legal because of the explicit cast to Money, but notice that the compiler still helped complete the cast by inserting an implicit cast from float to double.

Although the convenience of allowing implicit conversions can be tempting, doing so can introduce hard-to-find bugs. The rule of thumb is to make all conversion constructors explicit except when you're sure that you want a specific conversion to occur implicitly.

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Conversion functions

Conversion functions define conversions from a user-defined type to other types. These functions are sometimes referred to as 'cast operators' because they, along with conversion constructors, are called when a value is cast to a different type. The following example demonstrates a conversion function that converts from the user-defined type, Money, to a built-in type, double:

Notice that the member variable amount is made private and that a public conversion function to type double is introduced just to return the value of amount. In the function display_balance, an implicit conversion occurs when the value of balance is streamed to standard output by using the stream insertion operator <<. Because no stream-insertion operator is defined for the user-defined type Money, but there is one for built-in type double, the compiler can use the conversion function from Money to double to satisfy the stream-insertion operator.

Conversion functions are inherited by derived classes. Conversion functions in a derived class only override an inherited conversion function when they convert to exactly the same type. For example, a user-defined conversion function of the derived class operator int does not override—or even influence—a user-defined conversion function of the base class operator short, even though the standard conversions define a conversion relationship between int and short.

Declaring conversion functions

The following rules apply to declaring a conversion function:

The target type of the conversion must be declared prior to the declaration of the conversion function. Classes, structures, enumerations, and typedefs cannot be declared within the declaration of the conversion function.
Conversion functions take no arguments. Specifying any parameters in the declaration is an error.
Conversion functions have a return type that is specified by the name of the conversion function, which is also the name of the conversion's target type. Specifying a return type in the declaration is an error.
Conversion functions can be virtual.
Conversion functions can be explicit.

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Explicit conversion functions

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When a conversion function is declared to be explicit, it can only be used to perform an explicit cast. This prevents functions that accept an argument of the conversion function's target type from also implicitly accepting arguments of the class type, and prevents instances of the target type from being copy-initialized from a value of the class type. The following example demonstrates how to define an explicit conversion function and the effect it has on what code is well-formed.

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Here the conversion function operator double has been made explicit, and an explicit cast to type double has been introduced in the function display_balance to perform the conversion. If this cast were omitted, the compiler would be unable to locate a suitable stream-insertion operator << for type Money and an error would occur.

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