Research on the strange behavior of C ++ copy constructor

C ++ primer (4 edition) Chinese Version p407 said:"During replication initialization, you first use the specified constructor to create a temporary object, and then use the replication constructor to copy the temporary object to the object being created ."However, my lab results show thatThe replication constructor is not called.

I did an experiment (experiment 1). I first wrote a simple class that provides a default constructor and a constructor that accepts a single parameter, A copy constructor and A = overload function print a message in each of the above functions, prompting that the function is being called.

Next, use this class in the main function and use replication to initialize the class object. As stated in the book, the displayed result should be to first call the constructor of a single parameter to create a temporary object, output a prompt, then call the copy constructor, and output a prompt. But there is only one prompt! It is displayed that only the constructors that call the single-form parameters directly create an object and there is no temporary object. The results are weird!

Since I didn't call the copy constructor, I did another experiment (Experiment 2)Copy the constructor to privateIf no changes are made elsewhere and the same experiment is performed, the compilation fails. The error message is that my main function calls the copy constructor. Is the result very strange?

Problem: Does C ++ create a temporary object during replication initialization? If so, why is the result of experiment 1 displayed? If not, why is the result of experiment 2 displayed?

The class definition code of Experiment 1 is as follows:

class Employee{public:    Employee();    Employee(std::string sname);    // copy constructor    Employee(const Employee &old);    Employee& operator=(const Employee&old);     int getID() const { return id; }    std::string getName() const { return name;} private:    static int newid;    int id;    std::string name;};

The class definition code of Experiment 2 is as follows:

class Employee{public:    Employee();    Employee(std::string sname);    Employee& operator=(const Employee&old);     int getID() const { return id; }    std::string getName() const { return name;} private:    // copy constructor    Employee(const Employee &old);    static int newid;    intid;    std::string name;};

The class implementation code of the experiment is as follows:

int Employee::newid =1; Employee::Employee(): id(newid){    std::cout << "In defaultconstructor, id = " << id << std::endl;    ++newid;} Employee::Employee(std::stringsname) : id(newid), name(sname){    std::cout << "In singleparameter constructor, id = " << id << std::endl;    ++newid;}// copy constructorEmployee::Employee(constEmployee &old) : id(newid), name(old.getName()){    std::cout << "In copy constructor,id = " << id << std::endl;    ++newid;} Employee&Employee::operator=(const Employee &old){    std::cout << "In aissignfunction, id = " << id << std::endl;    name = old.getName();    return *this;}

The main function uses the following code:

int main(){    string name = "adu";    Employee em5 = name;     return 0;}

Result 1:

In single parameterconstructor, id = 1

Experiment 2 results:

Error: 'employee: employee (const employee &) 'is private

The issue was posted on the csdn Forum. You are welcome to discuss it together.