Std::make_unique,std::make_unique_default_init (3) Linux Manual Page

std::make_unique,std::make_unique_default_init – std::make_unique,std::make_unique_default_init

Synopsis

Defined in header<memory>

    template <class T, class... Args>
    (1)(since C++ 14)

        unique_ptr<T> make_unique(Args &&...args);                               (only for non-array types)

template< class T >                                                    (2) (since C++14)

unique_ptr<T> make_unique( std::size_t size );                             (only for array types with unknown bound)

template< class T, class... Args >                                     (3) (since C++14)

/* unspecified */ make_unique( Args&&... args ) = delete;                  (only for array types with known bound)

template< class T >                                                    (4) (since C++20)

unique_ptr<T> make_unique_default_init( );                                 (only for non-array types)

template< class T >                                                    (5) (since C++20)

unique_ptr<T> make_unique_default_init( std::size_t size );                (only for array types with unknown bound)

template< class T, class... Args >                                     (6) (since C++20)

/* unspecified */ make_unique_default_init( Args&&... args ) = delete;     (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 only participates in overload resolution 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 unknown bound T. This overload only participates in overload resolution if T is an array of unknown bound. The function is equivalent to:
  unique_ptr<T>(new typename std::remove_extent<T>::type[size]())
3,6) Construction of arrays of known bound is disallowed.
4) Same as (1), except that the object is default-initialized. This overload only participates in overload resolution 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 only participates in overload resolution if T is an array of unknown bound. The function is equivalent to:
  unique_ptr<T>(new typename std::remove_extent<T>::type[size])

Parameters

args – list of arguments with which an instance of T will be constructed.
size – the size 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

  // note: this implementation does not disable this overload for array types
  template<typename T, typename… Args>
  std::unique_ptr<T> make_unique(Args&&… args)
  {
      return std::unique_ptr<T>(new T(std::forward<Args>(args)…));
  }

Notes

Unlike std::make_shared (which has std::allocate_shared), std::make_unique does not have an allocator-aware counterpart. A hypothetical allocate_unique would be required to invent the deleter type D for the unique_ptr<T,D> it returns which would contain an allocator object and invoke both destroy and deallocate in its operator().

Example

// Run this code

#include <iostream>

#include <memory>
struct Vec3

{
    int x, y, z;
    Vec3()
        : x(0), y(0), z(0)
    {
    }
    Vec3(int x, int y, int z)
        : x(x), y(y), z(z)
    {
    }
    friend std::ostream &operator<<(std::ostream &os, 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):   " << '\n';
    for (int i = 0; i < 5; i++) {
        std::cout << "     " << 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}

std::make_unique,std::make_unique_default_init (3) Linux Manual Page