std::make_unique,std::make_unique_for_overwrite(3)

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