NAME¶

std::make_unique,std::make_unique_for_overwrite - std::make_unique,std::make_unique_for_overwrite

Synopsis¶

Defined in header <memory>
(since
C++14)
template< class T, class... Args > (until
unique_ptr<T> make_unique( Args&&... C++23)
args ); (only for
non-array
types)
(since
template< class T, class... Args > C++23)
constexpr unique_ptr<T> make_unique( (only for
Args&&... args ); non-array
types)
(since
C++14)
(until
template< class T > C++23)
unique_ptr<T> make_unique( (only for
std::size_t size ); array
types
with
unknown
bound)
(since
C++23)
template< class T > (only for
constexpr unique_ptr<T> make_unique( array
std::size_t size ); types
with
unknown
bound)
(since
C++14)
template< class T, class... Args > (only for
/* unspecified */ make_unique( (3) array
Args&&... args ) = delete; types
with
known
(1) bound)
(since
C++20)
template< class T > (until
unique_ptr<T> C++23)
make_unique_for_overwrite( ); (only for
(2) non-array
types)
(since
template< class T > C++23)
constexpr unique_ptr<T> (only for
make_unique_for_overwrite( ); non-array
types)
(since
C++20)
(until
template< class T > C++23)
unique_ptr<T> (only for
make_unique_for_overwrite( array
std::size_t size ); (4) types
with
unknown
bound)
(since
C++23)
template< class T > (5) (only for
constexpr unique_ptr<T> array
make_unique_for_overwrite( types
std::size_t size ); with
unknown
bound)
(since
C++20)
template< class T, class... Args > (only for
/* unspecified */ (6) array
make_unique_for_overwrite( Args&&... types
args ) = delete; with
known
bound)

Constructs an object of type T and wraps it in a std::unique_ptr.

1) 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)...))

2) 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.
4) 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)

5) 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

First version¶

// 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;

Second version¶

// 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: __cpp_lib_make_unique

Feature-test macro: __cpp_lib_smart_ptr_for_overwrite (for overloads (4-6))

Feature-test macro: __cpp_lib_constexpr_memory ((C++23) constexpr for overloads
(1,2,4,5))

Example¶

This section is incomplete
Reason: add make_unique_for_overwrite() demo

// Run this code

#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';
}
}

Output:¶

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 }

See also¶

constructor constructs a new unique_ptr
(public member function)
make_shared creates a shared pointer that manages a new object
make_shared_for_overwrite (function template)
(C++20)