Difference between revisions of "cpp/utility/declval"
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| − | Converts any type {{tt|T}} to a reference type, making it possible to use member functions in {{c|decltype}} expressions without specifying constructors. It 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. {{c|std::declval}} can only be used in unevaluated contexts, it is an error to evaluate an expression that contains this function. | + | Converts any type {{tt|T}} to a reference type, making it possible to use member functions in {{c|decltype}} expressions without specifying constructors. It 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, no definition exists for {{c|std::declval}} function. So, {{c|std::declval}} can only be used in unevaluated contexts, it is an error to evaluate an expression that contains this function. |
===Parameters=== | ===Parameters=== | ||
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===Exceptions=== | ===Exceptions=== | ||
{{noexcept}} | {{noexcept}} | ||
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| + | ===Explanation=== | ||
| + | Since declvalue converts the function type to rvalue reference, compiler will not evaluate it. | ||
===Example=== | ===Example=== | ||
Revision as of 22:01, 29 January 2013
Template:ddcl list begin <tr class="t-dsc-header">
<td><utility>
<td></td> <td></td> </tr> <tr class="t-dcl ">
<td >typename std::add_rvalue_reference<T>::type declval();
<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list end
Converts any type T to a reference type, making it possible to use member functions in decltype expressions without specifying constructors. It 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, no definition exists for std::declval function. So, std::declval can only be used in unevaluated contexts, it is an error to evaluate an expression that contains this function.
Contents |
Parameters
(none)
Return value
Cannot be called, thus never returns a value, but the return type is T&& unless T is an lvalue reference type, in which case T& is returned.
Exceptions
Explanation
Since declvalue converts the function type to rvalue reference, compiler will not evaluate it.
Example
#include <utility> #include <iostream> struct Default { int foo() const {return 1;} }; struct NonDefault { NonDefault(const NonDefault&) {} int foo() const {return 1;} }; int main() { decltype(Default().foo()) n1 = 1; // int n1 // decltype(NonDefault().foo()) n2 = n1; // will not compile decltype(std::declval<NonDefault>().foo()) n2 = n1; // int n2 std::cout << "n2 = " << n2 << '\n'; }
Output:
n2 = 1
See also
| (C++11)(removed in C++20)(C++17) |
deduces the result type of invoking a callable object with a set of arguments (class template) |
