STD：移动

// Simple move constructor
A(A&& arg) : member(std::move(arg.member)) // the expression "arg.member" is lvalue
{} 
// Simple move assignment operator
A& operator=(A&& other) {
     member = std::move(other.member
     return *this;
}

std::vector<std::string> v;
std::string str = "example";
v.push_back(std::move(str) // str is now valid but unspecified
str.back( // undefined behavior if size() == 0: back() has a precondition !empty()
str.clear( // OK, clear() has no preconditions

std::vector<int> v = {2, 3, 3};
v = std::move(v // undefined behavior

#include <iostream>
#include <utility>
#include <vector>
#include <string>
 
int main()
{
    std::string str = "Hello";
    std::vector<std::string> v;
 
    // uses the push_back(const T&) overload, which means 
    // we'll incur the cost of copying str
    v.push_back(str
    std::cout << "After copy, str is \"" << str << "\"\n";
 
    // uses the rvalue reference push_back(T&&) overload, 
    // which means no strings will be copied; instead, the contents
    // of str will be moved into the vector.  This is less
    // expensive, but also means str might now be empty.
    v.push_back(std::move(str)
    std::cout << "After move, str is \"" << str << "\"\n";
 
    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";
}

After copy, str is "Hello"
After move, str is ""
The contents of the vector are "Hello", "Hello"

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