CODEWITHSUNDEEP
c++oopmove-semanticsmove-constructor
hacker_panda
hacker_panda

Reputation: 71

Move ctor is not getting invoked

Below is my code 

#include<iostream>
#include<string.h>
using namespace std;

class mystring {
    private:
        char * ptr;
    public:
        mystring() {
            ptr = new char[1];
            ptr[0] = '\0';
        }
        mystring(const char * obj) {
            cout<<"param called "<<endl;
            int len = strlen(obj);
            ptr = new char[len+1];
            strcpy(ptr,obj);
        }
        mystring(mystring& obj) {
            cout<<"copy called "<<endl;
            ptr = new char[strlen(obj.ptr)+1];
            strcpy(ptr, obj.ptr);
        }
        mystring(mystring&& obj) noexcept{
            cout<<"shallow copy created "<<endl;
            ptr = obj.ptr;
            obj.ptr = NULL;
        }
        friend ostream& operator<<(ostream& out, mystring& obj) {
            out<<obj.ptr<<endl;
            return out;
        }
};

int main() {
    mystring s1 = move("Hello World");
    mystring s2 = s1;
    return 0;
}

When I say mystring s1 = move("Hello World"); My understanding is that move ctor should be invoked but for some reason the output of the above code is param called copy called. I am not sure why param ctor is getting invoked for that when I am trying to do a shallow copy. Can somebody help me understanding the output please. Thanks!

Upvotes: 0

Views: 56

Answers (1)

alfC
alfC

Reputation: 16242

In the line

mystring s1 = move("Hello World");

you are moveing the string literal "Hellow Word", not an object of type mystring.

int main() {
    mystring s1 = move("Hello World"); // moving a c-string, not very useful
    mystring s2 = s1; // calls copy constructor
    mystring s3 = std::move(s1); // new line, calls move constructor
    return 0;
}

will print

param called 
copy called 
shallow copy created

https://godbolt.org/z/cxh7qrzcd

Finally, note the the "shallow copy" is not at all a good description of what move doest.

Upvotes: 3

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