std::declval - cppreference.com (original) (raw)
| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ---------------------------------------------- | | template< class T > typename std::add_rvalue_reference<T>::type declval() noexcept; | | (since C++11) (until C++14) (unevaluated-only) | | template< class T > std::add_rvalue_reference_t<T> declval() noexcept; | | (since C++14) (unevaluated-only) |
Helper template for writing expressions that appear in unevaluated contexts, typically the operand of decltype. In unevaluated context, this helper template converts any type T (which may be an incomplete type) to an expression of that type, making it possible to use member functions of T without the need to go through constructors.
std::declval can only be used in unevaluated contexts and is not required to be defined; it is an error to evaluate an expression that contains this function. Formally, the program is ill-formed if this function is odr-used.
Contents
[edit] Parameters
(none)
[edit] Return value
Cannot be evaluated and thus never returns a value. The return type is T&& (reference collapsing rules apply) unless T is (possibly cv-qualified) void, in which case the return type is T.
[edit] Notes
std::declval is commonly used in templates where acceptable template parameters may have no constructor in common, but have the same member function whose return type is needed.
[edit] Possible implementation
template typename std::add_rvalue_reference::type declval() noexcept { static_assert(false, "declval not allowed in an evaluated context"); }
[edit] Example
#include #include struct Default { int foo() const { return 1; } }; struct NonDefault { NonDefault() = delete; int foo() const { return 1; } }; int main() { decltype(Default().foo()) n1 = 1; // type of n1 is int decltype(std::declval().foo()) n2 = 1; // same // decltype(NonDefault().foo()) n3 = n1; // error: no default constructor decltype(std::declval().foo()) n3 = n1; // type of n3 is int std::cout << "n1 = " << n1 << '\n' << "n2 = " << n2 << '\n' << "n3 = " << n3 << '\n'; }
Output: