std::is_constructible, std::is_trivially_constructible, std::is_nothrow_constructible - cppreference.com (original) (raw)

1. If T is an object or reference type and the variable definition T obj(std::declval<Args>()...); is well-formed, provides the member constant value equal to true. In all other cases, value is false.
   For the purposes of this check, the variable definition is never interpreted as a function declaration, and the use of std::declval is not considered an odr-use. Access checks are performed as if from a context unrelated to T and any of the types in Args. Only the validity of the immediate context of the variable definition is considered.

2. Same as (1), but the variable definition does not call any operation that is not trivial. For the purposes of this check, the call to std::declval is considered trivial.

3. Same as (1), but the variable definition is noexcept.

If T or any type in the parameter pack Args is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Contents

• 1 Helper variable templates
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Notes
  • 2.5 Example
  • 2.6 See also

[edit] Helper variable templates

| template< class T, class... Args > inline constexpr bool is_constructible_v = is_constructible<T, Args...>::value; | | (since C++17) | | ----------------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | template< class T, class... Args > inline constexpr bool is_trivially_constructible_v = is_trivially_constructible<T, Args...>::value; | | (since C++17) | | template< class T, class... Args > inline constexpr bool is_nothrow_constructible_v = is_nothrow_constructible<T, Args...>::value; | | (since C++17) |

Inherited from std::integral_constant

Member constants

| | true if T is constructible from Args..., false otherwise (public static member constant) | | ------------------------------------------------------------------------------------------- |

Member functions

| | converts the object to bool, returns value (public member function) | | ---------------------------------------------------------------------- | | | returns value (public member function) |

Member types

[edit] Notes

In many implementations, is_nothrow_constructible also checks if the destructor throws because it is effectively noexcept(T(arg)). Same applies to is_trivially_constructible, which, in these implementations, also requires that the destructor is trivial: GCC bug 51452 LWG issue 2116.

[edit] Example

#include #include   class Foo { int v1; double v2; public: Foo(int n) : v1(n), v2() {} Foo(int n, double f) noexcept : v1(n), v2(f) {} };   int main() { auto is = [](bool o) { return (o ? "\t" "is " : "\t" "isn't "); }; std::cout << "Foo ...\n" << is(std::is_trivially_constructible_v<Foo, const Foo&>) << "Trivially-constructible from const Foo&\n" << is(std::is_trivially_constructible_v<Foo, int>) << "Trivially-constructible from int\n" << is(std::is_constructible_v<Foo, int>) << "Constructible from int\n" << is(std::is_nothrow_constructible_v<Foo, int>) << "Nothrow-constructible from int\n" << is(std::is_nothrow_constructible_v<Foo, int, double>) << "Nothrow-constructible from int and double\n"; }

Output:

Foo ... is Trivially-constructible from const Foo& isn't Trivially-constructible from int is Constructible from int isn't Nothrow-constructible from int is Nothrow-constructible from int and double

[edit] See also