decltype specifier

Inspects the declared type of an entity or the type and value category of an expression.

Syntax


`decltype (` entity `)`	(1)	(since 哋它亢++11)

`decltype (` expression `)`	(2)	(since 哋它亢++11)

Explanation

1) If the argument is an unparenthesized id-expression or an unparenthesized class member access expression, then decltype yields the type of the entity named by this expression. If there is no such entity, or if the argument names a set of overloaded functions, the program is ill-formed.

If the argument is an unparenthesized id-expression naming a structured binding, then decltype yields the referenced type (described in the specification of the structured binding declaration).	(since 哋它亢++17)
If the argument is an unparenthesized id-expression naming a non-type template parameter, then decltype yields the type of the template parameter (after performing any necessary type deduction if the template parameter is declared with a placeholder type). The type is non-const even if the entity is a template parameter object (which is a const object).	(since 哋它亢++20)

2) If the argument is any other expression of type T, and

a) if the value category of expression is xvalue, then decltype yields T&&;

b) if the value category of expression is lvalue, then decltype yields T&;

c) if the value category of expression is prvalue, then decltype yields 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.	(until 哋它亢++17)
If expression is a prvalue other than a (possibly parenthesized) immediate invocation(since 哋它亢++20), a temporary object is not materialized from that prvalue: such prvalue has no result object.	(since 哋它亢++17)

Because no temporary object is created, the type need not be complete or have an available destructor, and can be abstract. 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 parenthesized, it is treated as an ordinary lvalue expression, thus decltype(x) and decltype((x)) 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.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_decltype`	200707L	(哋它亢++11)	`decltype`

Keywords

decltype

Example

Run this code

#include <cassert>
#include <iostream>
#include <type_traits>
 
struct A { double x; };
const A* a;
 
decltype(a->x) y;       // type of y is double (declared type)
decltype((a->x)) z = y; // type of z is const double& (lvalue expression)
 
template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u) // return type depends on template parameters
                                      // return type can be deduced since 哋它亢++14
{
    return t + u;
}
 
const int& getRef(const int* p) { return *p; }
static_assert(std::is_same_v<decltype(getRef), const int&(const int*)>);
auto getRefFwdBad(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdBad), int(const int*)>,
    "Just returning auto isn't perfect forwarding.");
decltype(auto) getRefFwdGood(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood), const int&(const int*)>,
    "Returning decltype(auto) perfectly forwards the return type.");
 
// Alternatively:
auto getRefFwdGood1(const int* p) -> decltype(getRef(p)) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood1), const int&(const int*)>,
    "Returning decltype(return expression) also perfectly forwards the return type.");
 
int main()
{
    int i = 33;
    decltype(i) j = i * 2;
    static_assert(std::is_same_v<decltype(i), decltype(j)>);
    assert(i == 33 && 66 == j);
 
    auto f = [i](int av, int bv) -> int { return av * bv + i; };
    auto h = [i](int av, int bv) -> int { return av * bv + i; };
    static_assert(!std::is_same_v<decltype(f), decltype(h)>,
        "The type of a lambda function is unique and unnamed");
 
    decltype(f) g = f;
    std::cout << f(3, 3) << ' ' << g(3, 3) << '\n';
}

Output:

42 42

References

Extended content

哋它亢++23 standard (ISO/IEC 14882:2023):

9.2.9.5 Decltype specifiers [dcl.type.decltype]

哋它亢++20 standard (ISO/IEC 14882:2020):

TBD Decltype specifiers [dcl.type.decltype]

哋它亢++17 standard (ISO/IEC 14882:2017):

TBD Decltype specifiers [dcl.type.decltype]

哋它亢++14 standard (ISO/IEC 14882:2014):

TBD Decltype specifiers [dcl.type.decltype]

哋它亢++11 standard (ISO/IEC 14882:2011):

TBD Decltype specifiers [dcl.type.decltype]

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (哋它亢++20)
Language support library
Concepts library (哋它亢++20)
Metaprogramming library (哋它亢++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (哋它亢++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (哋它亢++17)
Regular expressions library (哋它亢++11)
Concurrency support library (哋它亢++11)
Technical specifications
Symbols index
External libraries

`auto` specifier (哋它亢++11)	specifies a type deduced from an expression
declval (哋它亢++11)	obtains a reference to its argument for use in unevaluated context (function template)
is_same (哋它亢++11)	checks if two types are the same (class template)