Swap, move, forward, exchange of utility component learning

utility Is a practical component library , It includes some very useful functions , such as swap,move,forwardexchange etc. , This article will learn the basic use of these functions .

swap	Swap the values of two objects ( Function templates )
exchange (C++14)	Replace the argument with a new value , And return its previous value ( Function templates )
forward (C++11)	Forward a function argument ( Function templates )
move (C++11)	Get the right value reference ( Function templates )
move_if_noexcept (C++11)	If the move constructor does not throw, then get the right value reference ( Function templates )
as_const (C++17)	Get the actual parameters const quote ( Function templates )
declval (C++11)	Get the reference of its parameter , Used in the context of non evaluation ( Function templates )

1.swap Swap the values of two objects , Example ：

#include <iostream>
#include <string>
#include <utility>

using namespace std;

class Yak {
public:
    Yak(string name = "", int h = 0, int w = 0):
        m_name(name),
        m_height(h),
        m_weight(w)
    {
        cout << "constructe============== " << this << " name= " << name << " height= " << h << " weight= " << w << endl;
    }

    ~Yak(){ cout << "destructe~~~~~~~~~~~~~~~ " << this << " name= " << this->m_name << " height= " << m_height << " weight= " << m_weight << endl;}
    
    string m_name{""};
    int m_height{0};
    int m_weight{0};
};


int main()
{
    Yak yak1;
    yak1.m_name = "yak1";
    yak1.m_height = 10001;
    yak1.m_weight = 201;

    Yak yak2;
    yak2.m_name = "yak2";
    yak2.m_height = 10002;
    yak2.m_weight = 202;

    cout << "Yak1.name= " << yak1.m_name << " Yak2.name= " << yak2.m_name << " yak1 = " << &yak1 << " yak2= " << &yak2 << endl;
    cout << "============swap=============" << endl;
    std::swap(yak1, yak2); 
    cout << "Yak1.name= " << yak1.m_name << " Yak2.name= " << yak2.m_name << " yak1 = " << &yak1 << " yak2= " << &yak2 << endl;

    cout << "Hello World!" << endl;
    return 0;

}

Running results ：

From the running results ： There is an additional destructor in the exchange , Have you produced a temporary object

The implementation in the original code is T c(std::move(a)); a=std::move(b); b=std::move(c); From this original code, we can see that a temporary object will be generated during the exchange c.

2.move be used for instructions object t Sure “ Be moved ”, That is, from t Efficient resource transfer to another object . especially , std::move Generate parameters that identify them t Of Dead value expression . It is exactly equivalent to a reference to an R-value type static_cast .

  Example ：

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

using namespace std;


int main()
{


    string str = "Yak is mao niu";
    vector<std::string> vecStr;

    //  Use  push_back(const T&)  heavy load ,
    //  It means that we will bring replication  str  Cost of 
    vecStr.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";

    //  Use the right value reference  push_back(T&&)  heavy load ,
    //  Indicates that the string is not copied ; It is 
    // str  The content of is moved into  vector
    //  The cost is relatively low , But it also means  str  Now it may be empty .
    vecStr.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";

    std::cout << "The contents of the vector are \"" << vecStr[0]
                                         << "\", \"" << vecStr[1] << "\"\n";


    cout << "Hello World!" << endl;
    return 0;
}

From the above results ,move Value after original str The content is emptied .

Here is a class to demonstrate its value ：

#include <iostream>
#include <string>
#include <vector>
#include <list>
#include <utility>

using namespace std;

class Yak {
public:
    Yak(string name = "", int h = 0, int w = 0):
        m_name(name),
        m_height(h),
        m_weight(w)
    {
        cout << "constructe============== " << this << " name= " << name << " height= " << h << " weight= " << w << endl;
    }

    ~Yak(){ cout << "destructe~~~~~~~~~~~~~~~ " << this << " name= " << this->m_name << " height= " << m_height << " weight= " << m_weight << endl;}
    string m_name{""};
    int m_height{0};
    int m_weight{0};
};


int main()
{
    Yak yak1;
    yak1.m_name = "yak1";
    yak1.m_height = 10001;
    yak1.m_weight = 201;

    Yak yak2;
    yak2.m_name = "yak2";
    yak2.m_height = 10002;
    yak2.m_weight = 202;

    vector<Yak> vec;
    vec.reserve(16);  // Prevent reallocation when adding data 
    vec.push_back(yak1);
    cout << "Yak2.name= " << yak2.m_name << " Yak2.height= " << yak2.m_height << " yak2.weight = " << yak2.m_weight << endl;
    vec.push_back(std::move(yak2));
    cout << "Yak2.name= " << yak2.m_name << " Yak2.height= " << yak2.m_height << " yak2.weight = " << yak2.m_weight << endl;

    cout << "Hello World!" << endl;
    return 0;
}

Running results ：

  Why? move after Yak2 The value of the object has not been cleared ？？？

Reference resources ：

Tool library - cppreference.com