C++ std:move usage sample

cpp_move.cpp

#include <iostream>
#include <utility>
#include <vector>

using namespace std;

class SimpleString {
public:
	SimpleString(int s, const char* content): size(s+1), string(new char[s+1]) {
		for (int i = 0; i < size - 1; i++) {
			string[i] = content[i];
		}
		string[size-1] = '\0';
	}
	
	SimpleString(const SimpleString& a) {
		cout << "COPY cons get called" << endl;
		size = a.size;
		string = new char[size+1];
		for (int i = 0; i < size; i++) {
			string[i] = a.string[i];
		}
	}
	
	SimpleString(SimpleString && a): size(a.size) {
		cout << "MOVE cons get called" << endl;
		string = a.string;
		a.string = nullptr;
		a.size = 0;
	}
	
	static void test(const SimpleString & a) {
		cout << "test(const SimpleString& a) get called" << endl; 
	}
	
	static void test(SimpleString && a) {
		cout << "test(SimpleString && a) get called" << endl;
	}
	
	void print(const char * label) {
		cout << label << ":" << (size > 0 ? string : "<empty>") << ", size:" << size << endl;
	}
private:
	char *string = nullptr;
	int size = 0;
};


SimpleString tempString() {
	return SimpleString(5, "hello");
}

void printSeparateLine(string label) {
	cout << endl << "========= " << label << " ========" << endl;
}

int main(int argc, char *argv[]) {
	SimpleString a0(7, "abcdefg");
	
	printSeparateLine("SimpleString a1 = a0");

	SimpleString a1 = a0;
	a0.print("a0");
	a1.print("a1");
	
	printSeparateLine("SimpleString a2 = std::move(a1)");
	
	SimpleString a2 = std::move(a0);
	a0.print("a0");
	a2.print("a2");
	
	printSeparateLine("SimpleString a3 = static_cast<SimpleString&&> (a2)");
	
	SimpleString a3 = static_cast<SimpleString&&> (a2);
	a2.print("a2");
	a3.print("a3");
	
	printSeparateLine("SimpleString a4 = a1");
	SimpleString a4 = a1;
	SimpleString::test(a4);
	SimpleString::test(std::move(a4));  
	//std::move(a) 等价于 static_cast<SimpleString &&>(a), C++ doc:
	//In particular, std::move produces an xvalue expression that identifies its argument t. 
	//It is exactly equivalent to a static_cast to an rvalue reference type.
	SimpleString::test(static_cast<SimpleString&&>(a4));
	a4.print("a4");
	
	printSeparateLine("SimpleString a5 = tempString()");
	SimpleString::test(tempString()); //tempString() 返回的是一个右值
		
	printSeparateLine("\b=======================");

	printSeparateLine("test via vector.push_back()");
	vector<SimpleString> strings;
	strings.reserve(10);
	SimpleString a5(11, "hello world");
	
	cout << endl << "call strings.push_back(a5)" << endl;
	strings.push_back(a5);
	a5.print("a5");
	
	cout << endl << "call strings.push_back(std::move(a5))" << endl;
	strings.push_back(std::move(a5));
	a5.print("a5_after_move");
}

Output with clang compiler:

========= SimpleString a1 = a0 ========
COPY cons get called
a0:abcdefg, size:8
a1:abcdefg, size:8

========= SimpleString a2 = std::move(a1) ========
MOVE cons get called
a0:<empty>, size:0
a2:abcdefg, size:8

========= SimpleString a3 = static_cast<SimpleString&&> (a2) ========
MOVE cons get called
a2:<empty>, size:0
a3:abcdefg, size:8

========= SimpleString a4 = a1 ========
COPY cons get called
test(const SimpleString& a) get called
test(SimpleString && a) get called
test(SimpleString && a) get called
a4:abcdefg, size:8

========= SimpleString a5 = tempString() ========
test(SimpleString && a) get called

================================ ========

========= test via vector.push_back() ========

call strings.push_back(a5)
COPY cons get called
a5:hello world, size:12

call strings.push_back(std::move(a5))
MOVE cons get called
a5_after_move:<empty>, size:0