std::make_unique, std::make_unique_for_overwrite
From cppreference.com
< cpp | memory | unique ptr
| Defined in header <memory>
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| template< class T, class... Args > unique_ptr<T> make_unique( Args&&... args ); |
(1) | (since C++14) (only for non-array types) |
| template< class T > unique_ptr<T> make_unique( std::size_t size ); |
(2) | (since C++14) (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) |
| template< class T > unique_ptr<T> make_unique_for_overwrite( ); |
(4) | (since C++20) (only for non-array types) |
| template< class T > unique_ptr<T> make_unique_for_overwrite( std::size_t size ); |
(5) | (since C++20) (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.
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| 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
| First version |
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// 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; |
| Second version |
// 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 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 <iomanip> #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
constructs a new unique_ptr (public member function) | |
| creates a shared pointer that manages a new object (function template) |