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( C++23)
Args&&... args ); (only for
non-array
types)
(since
template< class T, class... Args > C++23)
constexpr unique_ptr<T> (only for
make_unique( 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> array
make_unique( 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
bound)
(since
(1) C++20)
template< class T > (until
unique_ptr<T> C++23)
make_unique_for_overwrite(); (only for
non-array
types)
(2) (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( types
Args&&... 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

make_unique (1-3)
// 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 (4-6)
// 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 Feature
__cpp_lib_make_unique 201304L (C++14) std::make_unique; overload (1)
Smart pointer creation with default
initialization
__cpp_lib_smart_ptr_for_overwrite 202002L (C++20) (std::allocate_shared_for_overwrite,
std::make_shared_for_overwrite,
std::make_unique_for_overwrite);
overloads (4-6)
__cpp_lib_constexpr_memory 202202L (C++23) constexpr for overloads (1,2,4,5)

Example¶

This section is incomplete
Reason: add more make_unique_for_overwrite() demos

// Run this code

#include <cstddef>
#include <iomanip>
#include <iostream>
#include <memory>
#include <utility>

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 << " }";
}
};

// Output Fibonacci numbers to an output iterator.
template<typename OutputIt>
OutputIt fibonacci(OutputIt first, OutputIt last)
{
for (int a = 0, b = 1; first != last; ++first)
{
*first = b;
b += std::exchange(a, b);
}
return first;
}

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);

// Create a unique_ptr to an uninitialized array of 10 integers,
// then populate it with Fibonacci numbers.
std::unique_ptr<int[]> i1 = std::make_unique_for_overwrite<int[]>(10);
fibonacci(i1.get(), i1.get() + 10);

std::cout << "make_unique<Vec3>(): " << *v1 << '\n'
<< "make_unique<Vec3>(0,1,2): " << *v2 << '\n'
<< "make_unique<Vec3[]>(5): ";
for (std::size_t i = 0; i < 5; ++i)
std::cout << std::setw(i ? 30 : 0) << v3[i] << '\n';
std::cout << '\n';

std::cout << "make_unique_for_overwrite<int[]>(10), fibonacci(...): [" << i1[0];
for (std::size_t i = 1; i < 10; ++i)
std::cout << ", " << i1[i];
std::cout << "]\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 }

make_unique_for_overwrite<int[]>(10), fibonacci(...): [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

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)

Category:¶

* Todo with reason