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std::make_shared, std::make_shared_for_overwrite - cppreference.com

template< class T, class... Args > shared_ptr<T> make_shared( Args&&... args );

(1) (since C++11) template< class T > shared_ptr<T> make_shared( std::size_t N ); (2) (since C++20)

template< class T > shared_ptr<T> make_shared();

(3) (since C++20) (4) (since C++20) (5) (since C++20)

template< class T > shared_ptr<T> make_shared_for_overwrite();

(6) (since C++20) template< class T > shared_ptr<T> make_shared_for_overwrite( std::size_t N ); (7) (since C++20)

Allocates memory for an object and initialize the object with the supplied arguments. Returns a std::shared_ptr object managing the newly created object.

1)

The object is of type

T

, and is constructed as if by

::new (pv) T(std::forward<Args>(args)...)

, where

pv

is a

void*

pointer to storage suitable to hold an object of type

T

. If the object is to be destroyed, it is destroyed as if by

pt->~T()

, where

pt

is a pointer to that object of type

T

This overload participates in overload resolution only if T is not an array type.

(since C++20)

This overload participates in overload resolution only if T is an unbounded array type.

3) The object is of type T. Each element has a default initial value.

This overload participates in overload resolution only if T is a bounded array type.

This overload participates in overload resolution only if T is an unbounded array type.

5) The object is of type T. Each element has the initial value u.

This overload participates in overload resolution only if T is a bounded array type.

6)

The object is of type

T

If T is not an array type, the object is constructed as if by ::new (pv) T, where pv is a void* pointer to storage suitable to hold an object of type T. If the object is to be destroyed, it is destroyed as if by pt->~T(), where pt is a pointer to that object of type T.
If T is a bounded array type, the initial value is unspecified for each element.

This overload participates in overload resolution only if T is not an array type or is an bounded array type.

7)

The object is of type

std::remove_extent_t<T>[N]

. The initial value is unspecified for each element.

This overload participates in overload resolution only if T is an unbounded array type.

Initializing and destroying array elements

Array elements of type U are initialized in ascending order of their addresses.

If U is not an array type, each element is constructed as if by the following expression, where pv is a void* pointer to storage suitable to hold an object of type U:

2,3) ::new (pv) U()

4,5) ::new (pv) U(u)

6,7) ::new (pv) U

Otherwise, recursively initializes the elements of each element. For the next dimension:

U becomes std::remove_extent_t<U>.
For overloads (4,5), u becomes the corresponding element of u.

When the lifetime of the object managed by the return std::shared_ptr ends, or when the initialization of an array element throws an exception, the initialized elements are destroyed in the reverse order of their original construction.

For each array element of non-array type U to be destroyed, it is destroyed as if by pu->~U(), where pu is a pointer to that array element of type U.

(since C++20) [edit] Parameters args - list of arguments with which an object of T will be constructed N - array size to use u - the initial value to initialize every element of the array [edit] Return value

std::shared_ptr to an object of type T or std::remove_extent_t<T>[N] if T is an unbounded array type(since C++20).

For the returned std::shared_ptr r, r.get() returns a non-null pointer and r.use_count() returns 1.

[edit] Exceptions

May throw std::bad_alloc or any exception thrown by the constructor of T. If an exception is thrown, the functions have no effect. If an exception is thrown during the construction of the array, already-initialized elements are destroyed in reverse order.(since C++20)

[edit] Notes

These functions will typically allocate more memory than sizeof(T) to allow for internal bookkeeping structures such as reference counts.

These functions may be used as an alternative to std::shared_ptr<T>(new T(args...)). The trade-offs are:

std::shared_ptr<T>(new T(args...)) performs at least two allocations (one for the object T and one for the control block of the shared pointer), while std::make_shared<T> typically performs only one allocation (the standard recommends, but does not require this; all known implementations do this).
If any std::weak_ptr references the control block created by std::make_shared after the lifetime of all shared owners ended, the memory occupied by T persists until all weak owners get destroyed as well, which may be undesirable if sizeof(T) is large.
std::shared_ptr<T>(new T(args...)) may call a non-public constructor of T if executed in context where it is accessible, while std::make_shared requires public access to the selected constructor.
Unlike the std::shared_ptr constructors, std::make_shared does not allow a custom deleter.
std::make_shared uses ::new, so if any special behavior has been set up using a class-specific operator new, it will differ from std::shared_ptr<T>(new T(args...)).

code such as f(std::shared_ptr<int>(new int(42)), g()) can cause a memory leak if g gets called after new int(42) and throws an exception, while f(std::make_shared<int>(42), g()) is safe, since two function calls are never interleaved.

(until C++17)

A constructor enables shared_from_this with a pointer ptr of type U* means that it determines if U has an unambiguous and accessible(since C++17) base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates if (ptr != nullptr && ptr->weak_this .expired()) ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>> (*this, const_cast<std::remove_cv_t<U>*>(ptr));.

The assignment to the weak_this is not atomic and conflicts with any potentially concurrent access to the same object. This ensures that future calls to shared_from_this() would share ownership with the std::shared_ptr created by this raw pointer constructor.

The test ptr->weak_this .expired() in the code above makes sure that weak_this is not reassigned if it already indicates an owner. This test is required as of C++17.

[edit] Example

#include <iostream>
#include <memory>
#include <type_traits>
#include <vector>
 
struct C
{
    // constructors needed (until C++20)
    C(int i) : i(i) {}
    C(int i, float f) : i(i), f(f) {}
    int i;
    float f{};
};
 
int main()
{
    // using âautoâ for the type of âsp1â
    auto sp1 = std::make_shared<C>(1); // overload (1)
    static_assert(std::is_same_v<decltype(sp1), std::shared_ptr<C>>);
    std::cout << "sp1->{ i:" << sp1->i << ", f:" << sp1->f << " }\n";
 
    // being explicit with the type of âsp2â
    std::shared_ptr<C> sp2 = std::make_shared<C>(2, 3.0f); // overload (1)
    static_assert(std::is_same_v<decltype(sp2), std::shared_ptr<C>>);
    static_assert(std::is_same_v<decltype(sp1), decltype(sp2)>);
    std::cout << "sp2->{ i:" << sp2->i << ", f:" << sp2->f << " }\n";
 
    // shared_ptr to a value-initialized float[64]; overload (2):
    std::shared_ptr<float[]> sp3 = std::make_shared<float[]>(64);
 
    // shared_ptr to a value-initialized long[5][3][4]; overload (2):
    std::shared_ptr<long[][3][4]> sp4 = std::make_shared<long[][3][4]>(5);
 
    // shared_ptr to a value-initialized short[128]; overload (3):
    std::shared_ptr<short[128]> sp5 = std::make_shared<short[128]>();
 
    // shared_ptr to a value-initialized int[7][6][5]; overload (3):
    std::shared_ptr<int[7][6][5]> sp6 = std::make_shared<int[7][6][5]>();
 
    // shared_ptr to a double[256], where each element is 2.0; overload (4):
    std::shared_ptr<double[]> sp7 = std::make_shared<double[]>(256, 2.0);
 
    // shared_ptr to a double[7][2], where each double[2]
    // element is {3.0, 4.0}; overload (4):
    std::shared_ptr<double[][2]> sp8 = std::make_shared<double[][2]>(7, {3.0, 4.0});
 
    // shared_ptr to a vector<int>[4], where each vector
    // has contents {5, 6}; overload (4):
    std::shared_ptr<std::vector<int>[]> sp9 =
        std::make_shared<std::vector<int>[]>(4, {5, 6});
 
    // shared_ptr to a float[512], where each element is 1.0; overload (5):
    std::shared_ptr<float[512]> spA = std::make_shared<float[512]>(1.0);
 
    // shared_ptr to a double[6][2], where each double[2] element
    // is {1.0, 2.0}; overload (5):
    std::shared_ptr<double[6][2]> spB = std::make_shared<double[6][2]>({1.0, 2.0});
 
    // shared_ptr to a vector<int>[4], where each vector
    // has contents {5, 6}; overload (5):
    std::shared_ptr<std::vector<int>[4]> spC =
        std::make_shared<std::vector<int>[4]>({5, 6});
}

Output:

sp1->{ i:1, f:0 }
sp2->{ i:2, f:3 }

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 4024 C++20 it was unclear how the objects constructed in
std::make_shared_for_overwrite are destroyed made clear [edit] See also

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