initialise object with equal operator

Question

In the class defined as below named foo

class foo{
private:
    string str;
public:
    foo operator = (string s){
         str = s;
    }
};

int main(){
    foo a = "this initialization throwing an error";
    foo b;
    b = "but assignment after declaration is working fine";
}

error: conversion from 'const char [38]' to non-scalar type 'foo' requested

The above error is caused only when I am assigning value to the object instance with declaration but if I am assigning separately from declaration then the overloaded equal = operator is working fine.

I want in any method to assign an string to the object with equal operator and as a declaration like foo a = "abcd";

Answer 1

When you have

type name = something;

You are not doing assignment but instead you have copy-initialization (do not that there can be a move even though it is called copy). This means that you need a constructor in your class whose parameter matches that of the thing on the right hand side of the =.

In this case you do not have a constructor that takes a std::string, const char* or const char[] so the compiler is unable to construct an instance there.

The reason the second case works is that

b = "but assignment after declaration is working fine";

Is not trying to construct anything and therefore it calls the assignment operator which does work because

"but assignment after declaration is working fine"

can be converted to a std::string.

If you want your class to be constructable from a string literal or cstring then you can add a constructor to it in the form of

foo(const char* str) : str(str) {}