std::is_move_constructible, std::is_trivially_move_constructible, std::is_nothrow_move_constructible

Since C++11

Header: <type_traits>

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

# Declarations

template< class T >
struct is_move_constructible;

(since C++11)

template< class T >
struct is_trivially_move_constructible;

(since C++11)

template< class T >
struct is_nothrow_move_constructible;

(since C++11)

# Notes

Types without a move constructor, but with a copy constructor that accepts const T& arguments, satisfy std::is_move_constructible.

Move constructors are usually noexcept, since otherwise they are unusable in any code that provides strong exception guarantee.

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

# Example

#include <string>
#include <type_traits>
 
struct Ex1
{
    std::string str; // member has a non-trivial but non-throwing move constructor
};
static_assert(std::is_move_constructible_v<Ex1>);
static_assert(!std::is_trivially_move_constructible_v<Ex1>);
static_assert(std::is_nothrow_move_constructible_v<Ex1>);
 
struct Ex2
{
    int n;
    Ex2(Ex2&&) = default; // trivial and non-throwing
};
static_assert(std::is_move_constructible_v<Ex2>);
static_assert(std::is_trivially_move_constructible_v<Ex2>);
static_assert(std::is_nothrow_move_constructible_v<Ex2>);
 
struct NoMove1
{
    // prevents implicit declaration of default move constructor;
    // however, the class is still move-constructible because its
    // copy constructor can bind to an rvalue argument
    NoMove1(const NoMove1&) {}
};
static_assert(std::is_move_constructible_v<NoMove1>);
static_assert(!std::is_trivially_move_constructible_v<NoMove1>);
static_assert(!std::is_nothrow_move_constructible_v<NoMove1>);
 
struct NoMove2
{
    // Not move-constructible since the lvalue reference
    // can't bind to the rvalue argument
    NoMove2(NoMove2&) {}
};
static_assert(!std::is_move_constructible_v<NoMove2>);
static_assert(!std::is_trivially_move_constructible_v<NoMove2>);
static_assert(!std::is_nothrow_move_constructible_v<NoMove2>);
 
int main() {}

# Defect reports

DRApplied toBehavior as publishedCorrect behavior
LWG 2196C++11the behavior was unclear if T&& cannot be formedthe value produced is false in this case

# See also