writing a c++ program using a cpp file with header file

Question

Just trying to write a basic c++ program for my college project

I declared a sample class called TestClass and put it in the testclass.h file in header files folder.

class TestClass
{
private:
    int an;
public:
    TestClass(void);
    int a[];
    void print();
    void compute();
    void get();
    ~TestClass(void);
};

And then to implement the above class, I created a new file in Source Files folders and then wrote this piece of code.

#include <iostream>
#include <conio.h>
#include "TestClass.h"



    TestClass:: vinayak(void)
    {

    }

    void TestClass:: print()
    {

    }
    void TestClass:: compute()
    {

    }
    void TestClass:: get()
    {

    }

    TestClass:: ~TestClass(void)
    {

    }

I then created a new file called Source.cpp and then wrote the piece of code.

#include "TestClass.h"
#include "TestClass.cpp"

void main()
{
    TestClass TObject = TestClass();
    TObject.get();

}

Here I am getting an error like below:

public: void __thiscall TestClass::compute(void)" already defined in Source.obj

Any ideas why ?

EDIT:

Based on @Marco's comment, I was able to compile the program. Also, I had to remove the constructor and the destructor declarations in the header file.

This made the program to work fine :)

It would be wise to call your variables something other than `TObject` if you are using C++Builder as it uses that name for a fundamental object. E.g . use something starting with a lower-case letter for a variable name. — M.M, Jul 26 '14 at 23:30

score 2 · Accepted Answer · answered Jul 26 '14 at 14:17

Every time you write

#include "something"

it's like copy-pasting the content of that "something" file into that specific location.

You're compiling a "TestClass" translation unit and a "Source" translation unit both containing the definition of your compute method.

Remove the cpp include in Source.cpp:

#include "TestClass.h"
// #include "TestClass.cpp" <- Remove this

void main()
{
    TestClass TObject = TestClass();
    TObject.get();

}

and the Source translation unit will know TestClass has a compute method, but the ODR (read it up) will be satisfied.

Summary of what you need to read about:

Difference between declaration / definition
What's a TU - translation unit
Difference between compiling and linking
What's the ODR and why it's important
What does the #include directive do

A: thanks Marco. I removed the `redefinition` and everything worked great. Thanks again :) — electronic_coder, Jul 26 '14 at 15:15

score 1 · Answer 2 · answered Jul 26 '14 at 14:20

1

Header file must be protected against multiple inclusions.

Add #pragma once on top of your header file, or change it to:

#ifndef TEST_CLASS_INCLUDE
#define TEST_CLASS_INCLUDE
class TestClass
{
...
};
#endif

Also, you should never include .cpp files. Just include include TestClass.h from Source.cpp and make sure TestClass.cpp is compiled and TestClass.o is linked.

Jean

answered Jul 26 '14 at 14:20

jpo38

20,821
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thank you. I have included `#pragma once` as you said. Thanks again. Not enough reputation to `upvote` :( But thanks again – electronic_coder Jul 26 '14 at 15:18

score 0 · Answer 3 · answered Jul 26 '14 at 14:17

0

You need to read up on linking

This line

#include "TestClass.cpp"

is not required

answered Jul 26 '14 at 14:17

Ed Heal

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score 0 · Answer 4 · answered Jul 26 '14 at 14:17

0

You are including the TestClass.cpp, you need to include only the header file

answered Jul 26 '14 at 14:17

vsoftco

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writing a c++ program using a cpp file with header file

4 Answers4