std:: make_unique, std:: make_unique_for_overwrite

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Utilities library
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(C++11) (until C++23)
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(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)



std::unique_ptr
Defined in header <memory>
(1)
template < class T, class ... Args >
unique_ptr < T > make_unique ( Args && ... args ) ;
(since C++14)
(until C++23)
(only for non-array types)
template < class T, class ... Args >
constexpr unique_ptr < T > make_unique ( Args && ... args ) ;
(since C++23)
(only for non-array types)
(2)
template < class T >
unique_ptr < T > make_unique ( std:: size_t size ) ;
(since C++14)
(until C++23)
(only for array types with unknown bound)
template < class T >
constexpr unique_ptr < T > make_unique ( std:: size_t size ) ;
(since C++23)
(only for array types with unknown bound)
template < class T, class ... Args >
/* unspecified */ make_unique ( Args && ... args ) = delete ;
(3) (since C++14)
(only for array types with known bound)
(4)
template < class T >
unique_ptr < T > make_unique_for_overwrite ( ) ;
(since C++20)
(until C++23)
(only for non-array types)
template < class T >
constexpr unique_ptr < T > make_unique_for_overwrite ( ) ;
(since C++23)
(only for non-array types)
(5)
template < class T >
unique_ptr < T > make_unique_for_overwrite ( std:: size_t size ) ;
(since C++20)
(until C++23)
(only for array types with unknown bound)
template < class T >
constexpr unique_ptr < T > make_unique_for_overwrite ( std:: size_t size ) ;
(since C++23)
(only for array types with unknown bound)
template < class T, class ... Args >
/* unspecified */ make_unique_for_overwrite ( Args && ... args ) = delete ;
(6) (since C++20)
(only for array types 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 )
__cpp_lib_smart_ptr_for_overwrite 202002L (C++20) Smart pointer creation with default initialization ( 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

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