Difference between revisions of "cpp/utility/declval"
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{{cpp/title|declval}} | {{cpp/title|declval}} | ||
{{cpp/utility/navbar}} | {{cpp/utility/navbar}} | ||
| − | {{ | + | {{dcl begin}} |
| − | {{ | + | {{dcl header|utility}} |
| − | {{ | + | {{dcl|since=c++11| |
template< class T > | template< class T > | ||
| − | typename std::add_rvalue_reference<T>::type declval(); | + | typename std::add_rvalue_reference<T>::type declval() noexcept; |
}} | }} | ||
| − | {{ | + | {{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|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|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 15: | Line 19: | ||
===Return value=== | ===Return value=== | ||
| − | Cannot be called | + | 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=== |
| − | {{noexcept}} | + | {{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 { | + | struct Default |
| − | int foo() const {return 1;} | + | { |
| + | int foo() const { return 1; } | ||
}; | }; | ||
| − | struct NonDefault { | + | struct NonDefault |
| − | NonDefault( | + | { |
| − | int foo() const {return 1;} | + | NonDefault() = delete; |
| + | int foo() const { return 1; } | ||
}; | }; | ||
int main() | int main() | ||
{ | { | ||
| − | decltype(Default().foo()) n1 = 1; // | + | decltype(Default().foo()) n1 = 1; // type of n1 is int |
| − | // decltype(NonDefault().foo()) n2 = n1; // | + | // decltype(NonDefault().foo()) n2 = n1; // error: no default constructor |
| − | decltype(std::declval<NonDefault>().foo()) n2 = n1; // | + | decltype(std::declval<NonDefault>().foo()) n2 = n1; // type of n2 is int |
| − | std::cout << "n2 = " << n2 << '\n'; | + | std::cout << "n1 = " << n1 << '\n' |
| + | << "n2 = " << n2 << '\n'; | ||
} | } | ||
| − | + | |output= | |
| + | n1 = 1 | ||
n2 = 1 | n2 = 1 | ||
}} | }} | ||
===See also=== | ===See also=== | ||
| − | {{ | + | {{dsc begin}} |
| − | {{ | + | {{dsc inc|cpp/language/dsc decltype}} |
| − | {{ | + | {{dsc inc|cpp/types/dsc result_of}} |
| − | {{ | + | {{dsc end}} |
| + | |||
| + | {{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) |
