# std::make_unique, std::make_unique_for_overwrite

From cppreference.com
< cpp‎ | memory‎ | 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 (1,2,3)
    // 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`; 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 make_unique_for_overwrite() demo

Run this code

    #include <iomanip>
    #include <iostream>
    #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 make_shared_for_overwrite (C++20) | creates a shared pointer that manages a new object (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/unique_ptr/make_unique&oldid=153959"