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std::make_shared

From cppreference.com
< cpp‎ | memory‎ | shared ptr
 
 
 
Dynamic memory management
Uninitialized storage
(C++17)
(deprecated since c++17)
(deprecated since c++17)
(deprecated since c++17)
Garbage collection support
Miscellaneous
(C++11)
(C++11)
C Library
Low level memory management
 
 
Defined in header <memory>
template< class T, class... Args >
shared_ptr<T> make_shared( Args&&... args );
(since C++11)

Constructs an object of type T and wraps it in a std::shared_ptr using args as the parameter list for the constructor of T.

The std::shared_ptr constructor called by this function enables shared_from_this with a pointer to the newly constructed object of type T.

Contents

[edit] Parameters

args - list of arguments with which an instance of T will be constructed.

[edit] Return value

std::shared_ptr of an instance of type T.

[edit] 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.

[edit] Notes

This function is typically used to replace the construction std::shared_ptr<T>(new T(args...)) of a shared pointer from the raw pointer returned by a call to new. In contrast to that expression, std::make_shared<T> typically allocates memory for the T object and for the std::shared_ptr's control block with a single memory allocation (this is a non-binding requirement in the Standard), where std::shared_ptr<T>(new T(args...)) performs at least two memory allocations.

Moreover, code such as f(std::shared_ptr<int>(new int(42)), g()) can cause a memory leak if g throws an exception because g() may be called after new int(42) and before the constructor of shared_ptr<int>. This doesn't occur in f(std::make_shared<int>(42), g()), since two function calls are never interleaved.

A constructor enables shared_from_this with a pointer ptr of type U* means that it determines if U has an unambiguous and accessible base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates the statement:

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));

Where weak_this is the hidden mutable std::weak_ptr member of std::shared_from_this. The assignment to the weak_this member 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 shared_ptr created by this raw pointer constructor.

The test ptr->weak_this.expired() in the exposition 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>
 
void foo(const std::shared_ptr<int>& i)
{
    (*i)++;
}
 
int main()
{
    auto sp = std::make_shared<int>(12);
    foo(sp);
    std::cout << *sp << std::endl;
}

Output:

13

[edit] See also

constructs new shared_ptr
(public member function) [edit]
creates a shared pointer that manages a new object allocated using an allocator
(function template) [edit]
creates a unique pointer that manages a new object
(function template) [edit]