Class don't work in c++ project when created in other files

Question

I made a class in a .cpp and a .h file with in the same project, and then I tried to make a calculating code. I made an object for it, with the name "co". I then tried to build it and run the file, and it showed nothing. Why is that happening and how can I try to fix it? The code:

main.cpp

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

int main()
{
  calculatorClass co ();

}

calculatorClass.cpp

#include "calculatorClass.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
calculatorClass :: calculatorClass ()
{

int x = 25;
int y = 37;
int z = 51;
int a = 14;
int b = 63;
int c = 75;
cout << x * y * z * a * b * c ;

}

calculatorClass.h

#ifndef CALCULATORCLASS_H
#define CALCULATORCLASS_H


class calculatorClass
{
    public:
        calculatorClass (int hello);

        calculatorClass();

    protected:

    private:
};

#endif // CALCULATORCLASS_H

Thanks!

`calculatorClass co ();` is a declaration of a function named `co` that takes no parameters and returns a `calculatorClass`. No instance of `calculatorClass` is created in your program. Make it `calculatorClass co;` — Igor Tandetnik, Jul 12 '20 at 03:55
Related: [https://www.fluentcpp.com/2018/01/30/most-vexing-parse/](https://www.fluentcpp.com/2018/01/30/most-vexing-parse/) — drescherjm, Jul 12 '20 at 04:01
Your code uses class contructors as if they are functions. Just write a function if that is what you want, there is no point in using a class in the way that you are. — john, Jul 12 '20 at 05:35
@john, I changed it a bit maybe he is just trying to understand constructors better. — Geno C, Jul 12 '20 at 05:40

score 0 · Answer 1 · edited Jul 19 '20 at 15:33

It looks like you are trying to create a constructor.

So the first thing to do is look at your calculatorClass.h. You need to change it to look like this because a constructor is a member function that has the same name as the class.

#ifndef CALCULATORCLASS_H
#define CALCULATORCLASS_H


class calculatorClass
{
public:
    calculatorClass(); //Constructor.

protected:

private:
};
#endif // CALCULATORCLASS_H

Then in your calculatorClass.cpp you will need to change it to this.The function header of a constructor's external definitions takes a form like this:

Classname::Classname(Parameters)

#include "calculatorClass.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
   calculatorClass::calculatorClass() // Changed.
{

    int x = 25;
    int y = 37;
    int z = 51;
    int a = 14;
    int b = 63;
    int c = 75;
    cout << x * y * z * a * b * c;

}

Then in your main.cpp file you are going to define an instance of the class calculatorClass which I did and named it calc. Now, the function calculatorClass is automatically called in main().

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

int main()
{
    calculatorClass Calc; // Define a calculatorClass object
    return 0;
}

Just a heads up, when you use ints they only hold 32 bits. So with all these large numbers you are trying to multiplying will roll over to a large negative value and you will not get the correct answer.

You might want to use long long int.

You should include explanations of what you changed and why those changes were needed. — underscore_d, Jul 12 '20 at 09:16
@Genoc You don't necessarily invoke the ctor by `calculatorClass Calc;` If you run valgrind on such a code, the object may appear uninitialized! — Kerek, Jul 19 '20 at 15:48

score 0 · Answer 2 · answered Jul 12 '20 at 12:54

change calculatorClass co (); to calculatorClass co. This will work. The explanation is as the following question - C++: warning: C4930: prototyped function not called (was a variable definition intended?).

By calling calculatorClass co (); you are not creating an object; you are declaring a function.

score 0 · Answer 3 · answered Jul 19 '20 at 15:42

The main issue in the code, that the following line is interpreted differently than you think:

calculatorClass calc();

This line is interpreted as a declaration of a function prototype instead of creating a new object of calculatorClass. This is known as the most vexing parse as noted in the comments.

There is a way to declare and initialize an object in a similar fashion:

calculatorClass calc{};

Notice that I have used {} and not () ! This was introduced in order to avoid this vexing parsing of the code.

In addition, I want to comment about some other bad habits in the code:

Using using namespace std; is a bad habit, which causes name pollution and may cause bugs with name ambiguity, and in the worst case related to unexpected functions being called!
Class names always start with a capital letter - calculatorClass should be Calculator (we know that it is a class, thus class shouldn't appear in the name!).
There is no need in the protected and private modifiers here.

Class don't work in c++ project when created in other files

3 Answers3