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# std::make_shared

From cppreference.com
< cpp‎ | memory‎ | shared ptr

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 object is constructed as if by the expression ::new (pv) T(std::forward<Args>(args)...), where `pv` is an internal `void*` pointer to storage suitable to hold an object of type `T`. The storage is typically larger than `sizeof(T)` in order to use one allocation for both the control block of the shared pointer and the `T` object.

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

### [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 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::shared_ptr supports array types (as of C++17), but `std::make_shared` does not. This functionality is supported by boost::make_shared

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 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

Run this code

    #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

(constructor) | constructs new `shared_ptr` (public member function) [edit]
allocate_shared | creates a shared pointer that manages a new object allocated using an allocator (function template) [edit]
make_unique (C++14) | creates a unique pointer that manages a new object (function template) [edit]

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