chrome源码之 Move Constructor

Move constructor is a idiom in C++ for efficiency( use move if possible instead of copy ), this article introduce the implement of move constructor in chrome source code ( move.h ).

Move constructor is used to transfer the ownership of a resource held by an object to another object in C++03 effectively.
Note: In C++11, move-constructors are implemented using the built-in rvalue reference feature.

An implemnt of `move constructor` in chrome:

// move.h
#ifndef BASEMOVE_H
#define BASEMOVE_H

#define MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
 private: \
struct rvalue_type { \
	explicit rvalue_type(type* object) : object(object) {} \
	type* object; \
}; \
	type(type&); \
	void operator=(type&); \
 public: \
 operator rvalue_type() { return rvalue_type(this); } \
 type Pass() { return type(rvalue_type(this)); } \
 private:

#endif//BASEMOVE_H

We see that the MOVE_ONLY_TYPE_FOR_CPP_03 macro has 4 parts:

rvalue_type typedef: can hold the host type
host defines private reference copy constructor and assign operator
host can auto convert to rvalue_type
Pass() method:
- host should implement constructor with argument of rvalue_type, which is the key of MOVE semantic
- Pass() return type(rvalue_type(this)) which is a temporary, it don’t match type&, so as to auto convert to rvalue_type

Example:

#include "move.h"

#include <vector>
#include <algorithm>
#include <iostream>

class Foo
{
	MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);

public:
	Foo(){}

	Foo(RValue other)
	{// finally using vector::swap
		// move ownership from other
		std::swap( m_vect, other.object->m_vect );
		std::swap( std::vector<int>(),
		    other.object->m_vect );
	}

	Foo& operator=(RValue rhs)
	{
		if ( this != rhs.object )
		{
			std::swap( m_vect, rhs.object->m_vect );
			std::swap( std::vector<int>(),
			    rhs.object->m_vect );
		}
		return *this;
	}

	void insert(int n)
	{
		m_vect.push_back(n);
	}

	void print()
	{
		std::for_each(m_vect.begin(), m_vect.end(), [=](int n)
		{
			std::cout << n << " ";
		});
		std::cout << std::endl;
	}

private:
	std::vector<int> m_vect;
};


int main()
{
	Foo a;
	a.insert(5);
	a.insert(6);

	Foo b;
	b.insert(7);

	b = a.Pass();

	std::cout << "a:\n";
	a.print();

	std::cout << "b:\n";
	b.print();

	return 0;
}

The Output:

b: 5 6

Thus, we see that the content of a has moved to b.

An implemnt of move constructor in chrome:

Example:

An implemnt of `move constructor` in chrome: