Including a cpp file that includes a header file [duplicate]

Question

I'm trying learn c++ due to school work. Recently I've come up with a problem that stuck with me for a while. If there were 3 files, main.cpp, Fish.cpp, Fish.h

Fish.h:

#ifndef FISH_H
#define FISH_H

class Fish
{
    public:
        Fish();
};

#endif

Fish.cpp:

#include <iostream>
#include Fish.h
using namespace std;

Fish::Fish()
{
    cout << "I love fish" << endl;
}

main.cpp :

#include <iostream>
#include "Fish.cpp"
using namespace std;

int main(){
    Fish Salmon;
    return 0;
}

Here comes the problem, I know if I want to use the Fish() constructor in "main.cpp", I'm supposed to include Fish.h, and then compile all 3 files, but due to curiosity, I tried to include "Fish.cpp". If I include "Fish.cpp" to "main.cpp", then everything in Fish.cpp will get copied into main.cpp, which includes the line <#include Fish.h>, right? But why every time I compile and run, the terminal gives me errors?

Please provide the error message you are getting. But in any case, never include .cpp files in other files. — walnut
– walnut, Commented Mar 25, 2020 at 3:57
The syntax for #include Fish.h is not correct in Fish.cpp, it should be #include "Fish.h"; is that the error you're getting? — nick
– nick, Commented Mar 25, 2020 at 3:59
"then compile all 3 files" -- no, you have only two source files to compile. Source files (.cpp) get compiled. Header files (.h) get #included (not vice versa). — JaMiT
– JaMiT, Commented Mar 25, 2020 at 4:00

Zem · Accepted Answer · 2020-03-25 04:01:11Z

0

include header file, NOT cpp files.

Fish.cpp

#include <iostream>
#include "Fish.h" // change here
using namespace std;

Fish::Fish()
{
    cout << "I love fish" << endl;
}

main.cpp

#include <iostream>
#include "Fish.h" // change here
using namespace std;

int main(){
    Fish Salmon;
    return 0;
}

answered Mar 25, 2020 at 4:01

Zem

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4 Comments

walnut Over a year ago

"I know [...] I'm supposed to include Fish.h, and then compile all 3 files, but due to curiosity, I tried to include "Fish.cpp"": OP knows that. That is not the question.

Zem Over a year ago

Thanks to help to understand question. Fish.cpp has Fish.h header include. So, if main.cpp can include Fish.cpp file, main.cpp includes Fish.h, Fish.cpp both, right?

Zem Over a year ago

I think this link will helpful. cplusplus.com/forum/general/39618. In here, Fish.cpp is actually being compiled twice, so there will be some problems. And, once the linker starts combining your object files, it picks up multiple definitions of whatever you had in Fish.cpp, and raises a few errors.

walnut Over a year ago

Yes, basically, that is very likely OP's issue.

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