std::make_unique, std::make_unique_for_overwrite

 

C++

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Feature test macros (C++20)

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Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) owner_hash (C++26) owner_equal (C++26) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler (C++11) set_new_handler launder (C++17) bad_alloc bad_array_new_length (C++11) nothrow_t align_val_t (C++17) destroying_delete_t (C++20) new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

std::unique_ptr

Member functions

unique_ptr::unique_ptr

unique_ptr::~unique_ptr

unique_ptr::operator=

Modifiers

unique_ptr::release

unique_ptr::reset

unique_ptr::swap

Observers

unique_ptr::get

unique_ptr::get_deleter

unique_ptr::operator bool

unique_ptr::operator*unique_ptr::operator->

unique_ptr::operator[]

Non-member functions

make_uniquemake_unique_for_overwrite (C++14)(C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(C++20)

operator<< (C++20)

swap(std::unique_ptr)

Helper classes

hash<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])

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)
__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

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_sharedmake_shared_for_overwrite (C++20)	creates a shared pointer that manages a new object (function template)