Does shared_ptr a = make_shared() create a copy of the shared_ptr before constructor is run?

Question

This might be a stupid question.

Let's say we're in C++11 land and we use make_shared() to create a smart pointer. We then use this smart pointer to initialize a variable with like this:

std::shared_ptr<class> = make_shared(/* args to c'tor of class*/ );

Now I know two things:

1. Assignement is not initialization. In this case we have initialisation. This would mean in the above case probably the copy constructor is called for the shared_ptr which is returned by make_shared.
2. Copy elision is only mandatory in C++17.

Does this mean that on every instance of make_shared a temporary copy of the shared_ptr is created and inserted into the copy constructor? Because this would mean for thread safety that a lock would have to be taken across the initialisation in case other threads preempt the thread and call shared_ptr::use_count() member function?

Answer 1

There are two things to avoid the copy:

• 1 is the compiler's RVO (return value optimization);
• 2 is the move constructor/assignment.

for code auto foo = std::make_shared<Foo>(); RVO will create the object directly on the stack. and even we disable the RVO by -fno-elide-constructors, the move constructor will try used as the returned object from make_shared is a temporary one.

Below is a simple test code. (this code only show the concept but not for a real-world shared_ptr implementation)

#include <iostream>

template <typename T>
struct my_shared_ptr
{
    T *t_{nullptr};

    my_shared_ptr(T *t): t_(t) {
        std::cout << "constructor" << std::endl;
    };

    my_shared_ptr(const my_shared_ptr<T>&) {
        std::cout << "copy" << std::endl;
    }

    my_shared_ptr<T>& operator=(const my_shared_ptr<T>&) {
        std::cout << "copy" << std::endl;
        return *this;
    }

#ifndef NO_MOVE
    my_shared_ptr(my_shared_ptr<T>&&) {
        std::cout << "move" << std::endl;
    }

    my_shared_ptr<T>& operator=(my_shared_ptr<T>&&) {
        std::cout << "move" << std::endl;
        return *this;
    }
#endif
};

template <typename T>
my_shared_ptr<T>
my_make_shared() {
    return my_shared_ptr<T>(new T);
}

struct Foo {};

int main()
{
    auto foo = my_make_shared<Foo>();
    return 0;
}

Condition 1, compile with c++11 shows:

$ g++ a.cc -std=c++11 ; ./a.out
constructor

Condition 2, compile with c++11/disable RVO shows:

$ g++ a.cc -std=c++11 -fno-elide-constructors ; ./a.out
constructor
move
move

Condition 3, compile with c++11/disable RVO/no move shows:

$ g++ a.cc -std=c++11 -fno-elide-constructors -DNO_MOVE ; ./a.out
constructor
copy
copy