class from a different file, variable declaration doesn't work "incomplete type is not allowed"

Question

I have a file like this:

#include <list>
#include "term.h"

class Term {

};

class Number : public Term {
private:
    double m_number;

    Term* m_exponent = nullptr;

public:
    Number(double number) {
        m_number = number;
    }

    Number(double number, Term* exponent) {
        m_number = number;
        m_exponent = exponent;
    }

    double getNumber() {
        return m_number;
    }

    void setExponent(Term* exponent) {
        m_exponent = exponent;
    }

    Term* getExponent() {
        return m_exponent;
    }
};

and a header file term.h like this:

#pragma once

class Term;

class Number;

Now in my main function, if I try to declare a Variable of type Number (Number a(10.0)), I get the following error: Incomplete Type is not allowed.

#include "term.h"

int main() {
   Number a(10.0);
}

I don't understand where this is coming from. Any help would be appreciated. Thanks in advance! :)

You want to place class definition in header file and class methods definitions in .cpp file. — Yksisarvinen, Feb 12 '21 at 12:53
Sorry I'm new to c++, so that question might sound stupid, but does class definition mean all the variables and function prototypes? — Reskareth, Feb 12 '21 at 13:02
To avoid asking possible "stupidly sound questions" and save time read one book of [The Definitive C++ Book Guide and List](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — 273K, Feb 12 '21 at 13:13

score 0 · Accepted Answer · answered Feb 12 '21 at 16:33

Okay, a quick tutorial.

C++ separates the concept of the definition of a class and the implementation. This makes it different from languages like Java. Now, you can, if you want, inline portions of your class in the definition, but you don't have to.

Your .h file should have things like:

class Foo { ... the full definition };

And then in your .cpp, you implement anything you didn't do in the .h:

void Foo::someMethod() { ... }

What is going on is that your main.cpp has absolutely no idea what a Number class is. It knows it's a class, but that's it. And it needs to know.

Basically, everything in your first snippet needs to appear in the .h file so that other .cpp files can see what it is.

class from a different file, variable declaration doesn't work "incomplete type is not allowed"

1 Answers1