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Difference between revisions of "cpp/memory/unique ptr/make unique"

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< cpp‎ | memory‎ | unique ptr
m (→‎Notes: rem)
m (→‎Notes: ftm)
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Unlike {{lc|std::make_shared}} (which has {{lc|std::allocate_shared}}), {{tt|std::make_unique}} does not have an allocator-aware counterpart. {{tt|allocate_unique}} proposed in {{wg21|P0211}} would be required to invent the deleter type {{tt|D}} for the {{c|std::unique_ptr<T,D>}} it returns which would contain an allocator object and invoke both {{tt|destroy}} and {{tt|deallocate}} in its {{c|operator()}}.
 
Unlike {{lc|std::make_shared}} (which has {{lc|std::allocate_shared}}), {{tt|std::make_unique}} does not have an allocator-aware counterpart. {{tt|allocate_unique}} proposed in {{wg21|P0211}} would be required to invent the deleter type {{tt|D}} for the {{c|std::unique_ptr<T,D>}} it returns which would contain an allocator object and invoke both {{tt|destroy}} and {{tt|deallocate}} in its {{c|operator()}}.
  
−
{{feature test macro|__cpp_lib_make_unique}}
+
{{ftm begin|std=1|comment=1}}
−
{{feature test macro|__cpp_lib_smart_ptr_for_overwrite|for overloads {{v|4-6}}}}
+
{{ftm|std=C++14|value=201304L|__cpp_lib_make_unique}}
−
{{feature test macro|__cpp_lib_constexpr_memory|{{mark|C++23}} {{tt|constexpr}} for overloads {{v|1,2,4,5}}}}
+
{{ftm|std=C++20|value=202002L|__cpp_lib_smart_ptr_for_overwrite|for overloads {{v|4-6}}}}
 +
{{ftm|std=C++23|value=202202L|__cpp_lib_constexpr_memory|{{tt|constexpr}} for overloads {{v|1,2,4,5}}}}
 +
{{ftm end}}
  
 
===Example===
 
===Example===

Revision as of 17:26, 9 August 2022

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(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])

Contents

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

First version 
// 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 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)
__cpp_lib_smart_ptr_for_overwrite 202002L (C++20) for 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 <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) [edit]
(C++20)
 creates a shared pointer that manages a new object
(function template) [edit]
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