std::make_unique, std::make_unique_for_overwrite
Declarations
- Array Types
- Non-Array Types
- Known Bounds
- Unknown Bounds
- C++23
- C++20
- C++14
// 3)
template< class T, class... Args >
/* unspecified */ make_unique( Args&&... args ) = delete;
// 6)
template< class T, class... Args >
/* unspecified */ make_unique_for_overwrite( Args&&... args ) = delete;
// 3)
template< class T, class... Args >
/* unspecified */ make_unique( Args&&... args ) = delete;
// 6)
template< class T, class... Args >
/* unspecified */ make_unique_for_overwrite( Args&&... args ) = delete;
// 3)
template< class T, class... Args >
/* unspecified */ make_unique( Args&&... args ) = delete;
- C++23
- C++20
- C++14
// 2)
template< class T >
constexpr unique_ptr<T> make_unique( std::size_t size );
// 5)
template< class T >
constexpr unique_ptr<T> make_unique_for_overwrite( std::size_t size );
// 2)
template< class T >
unique_ptr<T> make_unique( std::size_t size );
// 5)
template< class T >
unique_ptr<T> make_unique_for_overwrite( std::size_t size );
// 2)
template< class T >
unique_ptr<T> make_unique( std::size_t size );
- C++23
- C++20
- C++14
// 1)
template< class T, class... Args >
constexpr unique_ptr<T> make_unique( Args&&... args );
// 4)
template< class T >
constexpr unique_ptr<T> make_unique_for_overwrite( );
// 1)
template< class T, class... Args >
unique_ptr<T> make_unique( Args&&... args );
// 4)
template< class T >
unique_ptr<T> make_unique_for_overwrite( );
// 1)
template< class T, class... Args >
unique_ptr<T> make_unique( Args&&... args );
Constructs an object of type T
and wraps it in a std::unique_ptr
.
Constructs a non-array type T
. The arguments args are passed to the constructor of T
. This overload participates in overload resolution only if T
is not an array type.
The function is equivalent to:
unique_ptr<T>(new T(std::forward<Args>(args)...))
Constructs an array of the given dynamic size. The array elements are value-initialized. This overload participates in overload resolution only if T
is an array of
unknown bound. The function is equivalent to:
unique_ptr<T>(new std::remove_extent_t<T>[size]())
3,6)
Construction of arrays of known bound is disallowed.
Same as 1), except that the object is default-initialized. This overload participates in overload resolution only if T
is not an array type.
The function is equivalent to:
unique_ptr<T>(new T)
Same as 2), except that the array is default-initialized. This overload participates in overload resolution only if T
is an array of unknown bound.
The function is equivalent to:
unique_ptr<T>(new std::remove_extent_t<T>[size])
Parameters
args
- list of arguments with which an instance of T
will be constructed.
size
- the length of the array to construct
Return value
std::unique_ptr
of an instance of type T
.
Exceptions
May throw std::bad_alloc
or any exception thrown by the constructor of T
. If an exception is thrown, this function has no effect.
Possible Implementation
make_unique (1,2,3)
// C++14 make_unique
namespace detail {
template<class>
constexpr bool is_unbounded_array_v = false;
template<class T>
constexpr bool is_unbounded_array_v<T[]> = true;
template<class>
constexpr bool is_bounded_array_v = false;
template<class T, std::size_t N>
constexpr bool is_bounded_array_v<T[N]> = true;
} // namespace detail
template<class T, class... Args>
std::enable_if_t<!std::is_array<T>::value, std::unique_ptr<T>>
make_unique(Args&&... args)
{
return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
}
template<class T>
std::enable_if_t<detail::is_unbounded_array_v<T>, std::unique_ptr<T>>
make_unique(std::size_t n)
{
return std::unique_ptr<T>(new std::remove_extent_t<T>[n]());
}
template<class T, class... Args>
std::enable_if_t<detail::is_bounded_array_v<T>> make_unique(Args&&...) = delete;
make_unique_for_overwrite (4,5,6)
// C++20 make_unique_for_overwrite
template<class T>
requires (!std::is_array_v<T>)
std::unique_ptr<T> make_unique_for_overwrite()
{
return std::unique_ptr<T>(new T);
}
template<class T>
requires std::is_unbounded_array_v<T>
std::unique_ptr<T> make_unique_for_overwrite(std::size_t n)
{
return std::unique_ptr<T>(new std::remove_extent_t<T>[n]);
}
template<class T, class... Args>
requires std::is_bounded_array_v<T>
void make_unique_for_overwrite(Args&&...) = delete;
Notes
Unlike std::make_shared
(which has std::allocate_shared
), std::make_unique
does not have an allocator-aware counterpart. allocate_unique
proposed in
P0211
would be required to invent the deleter type D
for the std::unique_ptr<T,D>
it returns which would contain an allocator object and invoke both destroy and deallocate in its operator()
.
Feature-test macro | Value | Std | Comment |
---|---|---|---|
__cpp_lib_make_unique | 201304L | (C++14) | std::make_unique |
__cpp_lib_smart_ptr_for_overwrite | 202002L | (C++20) | for overloads (4-6) |
__cpp_lib_constexpr_memory | 202202L | (C++23) | constexpr for overloads (1,2,4,5 |
Example
#include <iostream>
#include <iomanip>
#include <memory>
struct Vec3
{
int x, y, z;
// following constructor is no longer needed since C++20
Vec3(int x = 0, int y = 0, int z = 0) noexcept : x(x), y(y), z(z) { }
friend std::ostream& operator<<(std::ostream& os, const Vec3& v)
{
return os << "{ x=" << v.x << ", y=" << v.y << ", z=" << v.z << " }";
}
};
int main()
{
// Use the default constructor.
std::unique_ptr<Vec3> v1 = std::make_unique<Vec3>();
// Use the constructor that matches these arguments
std::unique_ptr<Vec3> v2 = std::make_unique<Vec3>(0,1,2);
// Create a unique_ptr to an array of 5 elements
std::unique_ptr<Vec3[]> v3 = std::make_unique<Vec3[]>(5);
std::cout << "make_unique<Vec3>(): " << *v1 << '\n'
<< "make_unique<Vec3>(0,1,2): " << *v2 << '\n'
<< "make_unique<Vec3[]>(5): ";
for (int i = 0; i < 5; i++)
std::cout << std::setw(i ? 30 : 0) << v3[i] << '\n';
}
make_unique<Vec3>(): { x=0, y=0, z=0 }
make_unique<Vec3>(0,1,2): { x=0, y=1, z=2 }
make_unique<Vec3[]>(5): { x=0, y=0, z=0 }
{ x=0, y=0, z=0 }
{ x=0, y=0, z=0 }
{ x=0, y=0, z=0 }
{ x=0, y=0, z=0 }