1. Why do objects move
- Using moving instead of copying objects can greatly improve performance.
- Some resource objects that cannot be shared cannot be copied but can be moved. eg. IO class unique_ptr class
2. How to move objects
2.1 What is an rvalue?
"Lvalue is persistent; rvalue is short-lived" The lvalue has a persistent state, and the rvalue is either a literal constant or a temporary object created during expression evaluation.
2.2 Rvalue references
- Must be bound to an rvalue reference.
- Support mobile operation
- Smart binding to an object to be destroyed
- The code that uses rvalue references can freely take over the referenced object resources
int i = 42;
int &r = i; //正确，r是i的左值引用
int &r2 = i * 42; //错误，i*42是一个右值
const int &r3 = i * 42; //正确，可以将一个const的引用绑定到一个右值上
int &&rr = i; //错误，不能将一个右值引用绑定到一个左值上
int &&rr2 = i * 42; //正确，将rr2绑定到右值上
int &&rr1 = 42; //正确，字面常量是右值
int &&r2 = rr1; //错误，表达式rr1是左值
2.3 Standard library move function
The move function can be used to explicitly convert an lvalue to the corresponding rvalue reference type.
#include <utility>
int rr3 = std::move(rr1); //正确
2.4 Member functions to realize object movement
If a member function provides both copy and move versions, one version accepts an lvalue reference to const, and the second version accepts an rvalue reference to non-const.
void push_back(const X&); //拷贝，绑定到任意类型的X
void push_back(X&&); //移动，只能绑定到类型X的可修改右值
inline void StrVec::push_back(const std::string& s)
{
chk_n_alloc(); // ensure that there is room for another element
// construct a copy of s in the element to which first_free points
alloc.construct(first_free++, s);
}
inline void StrVec::push_back(std::string &&s)
{
chk_n_alloc(); // reallocates the StrVec if necessary
alloc.construct(first_free++, std::move(s));
}
Call example:
StrVec vec;
string s = "some string or another";
vec.push(s);//调用push(const string &)
vec.push("done");//调用push(string &&)