Linking two .cpp and a .h files

13
votes

I'm doing an exercise (from the third chapter of Thinking in C++) but I have a problem linking two .cpp files.

This is the exercise:

Create a header file (with an extension of ‘.h’). In this file, declare a group of functions by varying the argument lists and return values from among the following: void, char, int, and float. Now create a .cpp file that includes your header file and creates definitions for all of these functions. Each definition should simply print out the function name, argument list, and return type so you know it’s been called. Create a second .cpp file that includes your header file and defines int main( ), containing calls to all of your functions. Compile and run your program.

I've created the three files, but when I try to compile the compiler give me this error:

undefined reference to `func1()'

undefined reference to `func2(int)'|

undefined reference to `func3(char, int, double)'|

undefined reference to `func4()'|

||=== Build finished: 4 errors, 0 warnings ===|

Why it cannot found the function declaration?

##EDIT
These are my three files:

func_ex.h

void func1(void);
int func2(int i);
char func3(char c, int i, double d);
float func4(void);

func_ex.cpp

#include "func_ex.h"
using namespace std;

void func1(void) {
    cout << "void func1(void)" << endl;
}

int func2(int i) {
    cout << "int func2(int i)" << endl;
}

char func3(char c, int i, double d) {
    cout << "func3(char c, int i, double d)" << endl;
}

float func4(void) {
    cout << "func4(void)" << endl;
}

func_ex_main.cpp

#include "func_ex.h"
#include <iostream>
using namespace std;

int main(int argc, char* argv[]) {
    func1();
    int i;
    func2(i);
    char c; double d;
    func3(c, i, d);
    func4();
}

I'm usig GCC as compiler (and Code::Blocks as IDE, but I think that's not important).

8
c++linker
How are you trying to compile the program? It seems like you're not compiling/linking one of the files. - templatetypedef
Sounds like you are using the compiler wrong. The obvious explanation would be that you didn't tell the compiler about your first .cpp file, so it didn't find any of the function declarations in it. Post a few more details about exactly how you are using your compiler. - john
@unNaturhal: Are you sure both your cpp files are included in the Code::Blocks project? - Nemanja Trifunovic
Well your code is fine (ish). You must be using your IDE wrong. Sorry but I can't help there. - john
Actually the code you posted will not compile because you didn't have #include <iostream> in func_ex.cpp. It is possible an error in compiling func_ex.cpp is the root cause of the problem? Might explain why the linker fails to find the functions, if the file containg the functions has failed to compile. Pretty poor IDE if that's the case though. - john

8 Answers

10
votes

It sounds like the file is not finding the functions appropriately. Is the header file included in both files? You can include it like:

#include "myheader.h"

Did you make sure to compile both files together? Such as:

gcc -o myprogram file1.cpp file2.cpp

8
votes
gcc -c func_ex.cpp -o func_ex.o
gcc func_ex_main.cpp func_ex.o -o func_ex_main
6
votes

It looks every beginner is going through this (including myself), in the main.cpp you include the .cpp file and not the .h, 

//func_ex_main.cpp
#include "func_ex.cpp" // instead of .h
#include <iostream>
using namespace std;
//etc...

because the .cpp is linked to the header through the include in that .cpp file, and each function definition is linked to the deceleration through the :: notation before the definition.

Remember that the default constructor and destructor in the .h file is declared and defined automatically (if you didn't override or overload it), which will result in a compile error if you just re-defined it in the .cpp file, so if you didn't want to override it just don't mention the constructor neither the destructor in neither the header nor source files.

1
votes

For those like me who may be using the IDE BloodShed Dev C++.

Create a project folder then proceed to place your H files and your CPP files inside your project folder.

I had the same problem as mentioned above, the conclusion I came upon was that my compiler was only compiling the H files and wouldn't even read the CPP files linked to it. When I put them in a project folder my compiler would compile all files together hence allowing my code to run.

Here is a link of creating a project folder for those using Bloodshed.

http://www.horstmann.com/bigcpp/help/bloodshed/index.html

As for other IDE users, I believe the problem to be similar. IF anyone needs further elaboration just let me know.

0
votes

I use code::blocks too, and I had the same problem. The default setting in this IDE is that only one file is built. Here's what I did: project -> properties -> build targets -> Select the files related in the bottom right section. Hope this helps!

0
votes

First of all, how can you have func1(), func2(), func3() without having a return statement. Try to put your header body inside these (#include guards) :

#ifndef func_ex.h
#define func_ex.h
/* your code */

#endif
-1
votes

I think you should have #include"func_x.cpp" in your func_main.cpp

-3
votes

You need to put an include like this:

#include "headerfilename.h"

at the very top of each .cpp document.