Difference between revisions of "cpp/utility/declval"
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(rv. The whole point of declval is that not all types are default constructible) |
(→Possible implementation: simplify per CWG 2518) |
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{{cpp/utility/navbar}} | {{cpp/utility/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl header | utility}} | + | {{dcl header|utility}} |
| − | {{dcl | since=c++11 | | + | {{dcl|since=c++11| |
| − | template<class T> | + | template< class T > |
typename std::add_rvalue_reference<T>::type declval() noexcept; | typename std::add_rvalue_reference<T>::type declval() noexcept; | ||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| − | Converts any type {{tt|T}} to a reference type, making it possible to use member functions in {{ | + | Converts any type {{tt|T}} to a reference type, making it possible to use member functions in the operand of the [[cpp/language/decltype|{{tt|decltype}} specifier]] without the need to go through constructors. |
| − | {{tt|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. | + | {{tt|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. |
| − | Note that {{tt|declval}} can only be used in [[cpp/language/expressions# | + | Note that {{tt|std::declval}} can only be used in [[cpp/language/expressions#Potentially-evaluated expressions|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 [[cpp/language/definition#ODR-use|odr-used]]. |
===Parameters=== | ===Parameters=== | ||
| Line 19: | Line 19: | ||
===Return value=== | ===Return value=== | ||
| − | Cannot be called and thus never returns a value. The return type is {{tt|T&&}} unless {{tt|T}} is (possibly cv-qualified) {{ | + | Cannot be called and thus never returns a value. The return type is {{tt|T&&}} unless {{tt|T}} is (possibly cv-qualified) {{c/core|void}}, in which case the return type is {{tt|T}}.<!--Note that reference collapsing may take place when determine what T&& refers to.--> |
| + | |||
| + | ===Possible implementation=== | ||
| + | {{eq fun | ||
| + | |1= | ||
| + | template<typename T> | ||
| + | typename std::add_rvalue_reference<T>::type declval() noexcept | ||
| + | { | ||
| + | static_assert(false, "declval not allowed in an evaluated context"); | ||
| + | } | ||
| + | }} | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |code= | |
| − | + | ||
| − | + | ||
#include <iostream> | #include <iostream> | ||
| + | #include <utility> | ||
| − | struct Default { int foo() const { return 1; } }; | + | struct Default |
| + | { | ||
| + | int foo() const { return 1; } | ||
| + | }; | ||
struct NonDefault | struct NonDefault | ||
| Line 44: | Line 56: | ||
<< "n2 = " << n2 << '\n'; | << "n2 = " << n2 << '\n'; | ||
} | } | ||
| − | + | |output= | |
n1 = 1 | n1 = 1 | ||
n2 = 1 | n2 = 1 | ||
| Line 51: | Line 63: | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/language/dsc decltype}} | + | {{dsc inc|cpp/language/dsc decltype}} |
| − | {{dsc inc | cpp/types/dsc result_of}} | + | {{dsc inc|cpp/types/dsc result_of}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Latest revision as of 13:44, 21 September 2023
| Defined in header <utility>
|
||
| template< class T > typename std::add_rvalue_reference<T>::type declval() noexcept; |
(since C++11) | |
Converts any type T to a reference type, making it possible to use member functions in the operand of the decltype specifier without the need to go through constructors.
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.
Note that 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 called and thus never returns a value. The return type is T&& unless T is (possibly cv-qualified) void, in which case the return type is T.
[edit] Possible implementation
template<typename T> typename std::add_rvalue_reference<T>::type declval() noexcept { static_assert(false, "declval not allowed in an evaluated context"); } |
[edit] Example
Run this code
#include <iostream> #include <utility> 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(NonDefault().foo()) n2 = n1; // error: no default constructor decltype(std::declval<NonDefault>().foo()) n2 = n1; // type of n2 is int std::cout << "n1 = " << n1 << '\n' << "n2 = " << n2 << '\n'; }
Output:
n1 = 1 n2 = 1
[edit] See also
decltype specifier(C++11)
|
obtains the type of an expression or an entity |
| (C++11)(removed in C++20)(C++17) |
deduces the result type of invoking a callable object with a set of arguments (class template) |
