左值引用、右值引用、移动语义、完美转发，你知道的不知道的都在这里

int a = b + c; 

int a = 4; // a是左值，4作为普通字面量是右值

class A {
    xxx;
};
A a;
auto c = std::move(a); // c是将亡值
auto d = static_cast<A&&>(a); // d是将亡值

type &name = exp; // 左值引用
type &&name = exp; // 右值引用

int a = 5;
int &b = a; // b是左值引用
b = 4;
int &c = 10; // error，10无法取地址，无法进行引用
const int &d = 10; // ok，因为是常引用，引用常量数字，这个常量数字会存储在内存中，可以取地址

int a = 4;
int &&b = a; // error, a是左值
int &&c = std::move(a); // ok

class A {
public:
    A(int size) : size_(size) {
        data_ = new int[size];
    }
    A(){}
    A(const A& a) {
        size_ = a.size_;
        data_ = a.data_;
        cout << "copy " << endl;
    }
    ~A() {
        delete[] data_;
    }
    int *data_;
    int size_;
};
int main() {
    A a(10);
    A b = a;
    cout << "b " << b.data_ << endl;
    cout << "a " << a.data_ << endl;
    return 0;
}

class A {
public:
    A(int size) : size_(size) {
        data_ = new int[size];
    }
    A(){}
    A(const A& a) {
        size_ = a.size_;
        data_ = new int[size_];
        cout << "copy " << endl;
    }
    ~A() {
        delete[] data_;
    }
    int *data_;
    int size_;
};
int main() {
    A a(10);
    A b = a;
    cout << "b " << b.data_ << endl;
    cout << "a " << a.data_ << endl;
    return 0;
}

class A {
public:
    A(int size) : size_(size) {
        data_ = new int[size];
    }
    A(){}
    A(const A& a) {
        size_ = a.size_;
        data_ = new int[size_];
        cout << "copy " << endl;
    }
    A(A&& a) {
        this->data_ = a.data_;
        a.data_ = nullptr;
        cout << "move " << endl;
    }
    ~A() {
        if (data_ != nullptr) {
            delete[] data_;
        }
    }
    int *data_;
    int size_;
};
int main() {
    A a(10);
    A b = a;
    A c = std::move(a); // 调用移动构造函数
    return 0;
}

std::vector<string> vecs;
...
std::vector<string> vecm = std::move(vecs); // 免去很多拷贝

void PrintV(int &t) {
    cout << "lvalue" << endl;
}
void PrintV(int &&t) {
    cout << "rvalue" << endl;
}
template<typename T>
void Test(T &&t) {
    PrintV(t);
    PrintV(std::forward<T>(t));
    PrintV(std::move(t));
}
int main() {
    Test(1); // lvalue rvalue rvalue
    int a = 1;
    Test(a); // lvalue lvalue rvalue
    Test(std::forward<int>(a)); // lvalue rvalue rvalue
    Test(std::forward<int&>(a)); // lvalue lvalue rvalue
    Test(std::forward<int&&>(a)); // lvalue rvalue rvalue
    return 0;
}

std::vector<int> return_vector(void) {
    std::vector<int> tmp {1,2,3,4,5};
    return tmp;
}
std::vector<int> &&rval_ref = return_vector();

const std::vector<int>& rval_ref = return_vector();

std::vector<int>&& return_vector(void) {
    std::vector<int> tmp {1,2,3,4,5};
    return std::move(tmp);
}
std::vector<int> &&rval_ref = return_vector();

std::vector<int> return_vector(void)
{
    std::vector<int> tmp {1,2,3,4,5};
    return std::move(tmp);
}
std::vector<int> &&rval_ref = return_vector();

std::vector<int> return_vector(void)
{
    std::vector<int> tmp {1,2,3,4,5};
    return tmp;
}
std::vector<int> rval_ref = return_vector();