How to fix "cannot convert"?

Question

I got compile errors "cannot convert..", `"no instance of constructor..". I can't get what I did wrong. I tried to change the way of initialization of an object but it didn't help. Want to understand this better.

Error C2440 '': cannot convert from 'const char [11]' to 'Course'
Error C2512 'LectureTitle::LectureTitle': no appropriate default constructor available
Error (active) E0289 no instance of constructor "Course::Course" matches the argument list
Error (active) E0312 no suitable user-defined conversion from "Week" to "LectureTitle" exists

    #include <iostream>
    #include <string>
    #include <vector>
    using namespace std;
    struct Specialization {
        string data;
        explicit Specialization(string& new_value) {
            data = new_value;
        }
    };
    struct Course {
        string data;
        explicit Course(string& new_value) {
            data = new_value;
        }
    };
    struct Week {
        string data;
        explicit Week(string& new_value) {
            data = new_value;
        }
    };
    struct LectureTitle {
        string specialization;
        string course;
        string week;
        LectureTitle(Specialization& new_value1, Course& new_value2, Week new_value3) {
            specialization = new_value1.data;
            course = new_value2.data;
            week = new_value3.data;
        }
    };

    int main() {
        LectureTitle title (
            Specialization("C++"),
            Course("White belt"),
            Week("4th")
        );

    }

Give the FULL error please. It matters what X and Y are in the sentence "cannot convert X to Y". — JohnFilleau, Mar 14 '20 at 19:08
Please [edit] your question to include a complete and full copy-paste of the error output, and also add comments on the lines where you get the errors. — Some programmer dude, Mar 14 '20 at 19:09
By the way your code is using a MSVC specific non-portable extension that allows temporary variables to be bound to non-const references. Please try to avoid such things. — Some programmer dude, Mar 14 '20 at 19:10
@Someprogrammerdude btw where is I can switch off this extension? — Leeker, Mar 14 '20 at 19:28

score 1 · Accepted Answer · answered Mar 14 '20 at 21:53

LectureTitle title (
        Specialization("C++"),
        Course("White belt"),
        Week("4th")
    );

Here you are trying to create an instance of LectureTitle with temporary values (rvalues) of Specialization, Course and Week. They are temporary because they are not named variables, i.e. not defined as variables before this piece of code.

All your constructors, however, take named variables references (lvalues). That's why you get the compiler errors. To correct these errors you can make the constructors accept const value references like so:

explicit Specialization(const std::string& new_value) {
    data = new_value;
}

explicit Course(const std::string& new_value) {
    data = new_value;
}

explicit Week(const std::string& new_value) {
    data = new_value;
}

LectureTitle(const Specialization& new_value1, const Course& new_value2, const Week new_value3) {
    specialization = new_value1.data;
    course = new_value2.data;
    week = new_value3.data;
}

Const lvalue references can bind to rvalues - i.e. extend the lifetime of a temporary; and those temporaries can be used to intialise the member variables in your constructors.

How to fix "cannot convert"?

1 Answers1