Method parameter copied (not moved)

Question

I have the following code:

// header
	void Method1(const bool& b, std::vector<int>&& v);
	bool Method2();

bool CMyClass::Method1()
{
	std::vector<int> v{ 1, 2, 3 };
	TRACE("%d\n", v.size());
	Method2(true, std::move(v));
	TRACE("%d\n", v.size());
	return true;
}

void CMyClass::Method2(const bool& b, std::vector<int>&& v)
{
	TRACE("%d\n", v.size());
}

Outcome:

3

3

3

Why v is copied, but not moved?

Answer 1

std::move does not mean move action. It casts. signals that it can be moved

move

Unconditionally casts its argument to an rvalue reference, and thereby signals that it can be moved if its type is move-enabled.

You are just binding an rvalue reference to the vector you pass; not moving.

Compare the example:
Output:
3

temp is 3

v is 0

0

#include <iostream>
#include <vector>

std::vector<int> v{ 1,2,3 };
void Method2(const bool& b, std::vector<int> temp)
{

	std::cout << "temp is "<<temp.size()<<std::endl;
	std::cout <<"v is " <<v.size() << std::endl;


}

bool Method1()
{	
	std::cout << v.size() << std::endl;
	Method2(true, std::move(v));
	std::cout << v.size() << std::endl;

	return true;
}


int main()
{
	Method1();

}

Best regards,

Minxin Yu

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