decltype specifier

< cpp‎ | language

Inspects the declared type of an entity or queries the return type of an expression.


[edit] Syntax

decltype ( entity ) (1) (since C++11)
decltype ( expression ) (2) (since C++11)

[edit] Explanation

1) If the argument is either the unparenthesised name of an object/function, or is a member access expression (object.member or pointer->member), then the decltype specifies the declared type of the entity specified by this expression.
2) If the argument is any other expression of type T, then
a) if the value category of expression is xvalue, then the decltype specifies T&&
b) if the value category of expression is lvalue, then the decltype specifies T&
c) otherwise, decltype specifies T

If expression is a function call which returns a prvalue of class type or is a comma expression whose right operand is such a function call, a temporary object is not introduced for that prvalue. The class type need not be complete or have an available destructor. This rule doesn't apply to sub-expressions:in decltype(f(g())), g() must have a complete type, but f() need not.

Note that if the name of an object is parenthesised, it becomes an lvalue expression, thus decltype(arg) and decltype((arg)) are often different types.

decltype is useful when declaring types that are difficult or impossible to declare using standard notation, like lambda-related types or types that depend on template parameters.

[edit] Keywords


[edit] Example

#include <iostream>
struct A {
   double x;
const A* a = new A{0};
decltype( a->x ) x3;       // type of x3 is double (declared type)
decltype((a->x)) x4 = x3;  // type of x4 is const double& (lvalue expression)
template <class T, class U>
auto add(T t, U u) -> decltype(t + u); // return type depends on template parameters
int main() 
    int i = 33;
    decltype(i) j = i*2;
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';
    auto f = [](int a, int b) -> int {
       return a*b;
    decltype(f) f2 = f; // the type of a lambda function is unique and unnamed
    i = f(2, 2);
    j = f2(3, 3);
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';


i = 33, j = 66
i = 4, j = 9

[edit] See also

obtains the type of expression in unevaluated context
(function template)