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. 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. 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. Function local variablesĪ variable that is declared inside a function body is called a local variable or simply a local. 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. templateĪuto Add(const Lhs& lhs, const Rhs& rhs) -> decltype(lhs + rhs) Trailing return types are especially useful in function templates when the type of the return value depends on template parameters. A trailing return type is located on the right most side of the signature and is preceded by the -> operator.
Trailing return typesĪn "ordinary" return type is located on the left side of the function signature. 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.
Function return typesĪ 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.
Int DoMore(int num = 5, // Not a trailing parameter!įor more information, see Default Arguments. C2548: 'DoMore': missing default parameter for parameter 2 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. 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. Arguments of type void elsewhere in the list produce errors. T copy_object(T& obj) noexcept(std::is_pod) Ī 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. The values that are passed to the function are the arguments, whose types must be compatible with the parameter types in the function definition. The function can be invoked, or called, from any number of places in the program. The following function accepts two integers from a caller and returns their sum a and b are parameters of type int. Functions are useful for encapsulating common operations in a single reusable block, ideally with a name that clearly describes what the function does. A function can optionally return a value as output. A function can optionally define input parameters that enable callers to pass arguments into the function.
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A function is a block of code that performs some operation.