How to create a C++ library in Eclipse? - c++

Question

Here a person asks "How to make your own library in C++ ?". This is exactly the question that I have. There is an answer to this question:
Create the files "mylibrary.h" and "mylibrary.cpp".
Only compile the library, and it shouldn't have a "main()" function in
it. This step will generate an object file called "myfile.o".
Create another file called something like "main.cpp". In main.cpp
you need to #include "mylibrary.h".
Compile main.cpp and add the library from step 2 to link in (in this case "myfile.o"). If you don't add this library, you'll get a
error message like "linker error - can't find function definition", or
something like that.
But I still have some questions:
The Library.cpp should have a main() function. But what should be in this main function? I do not want my library to execute something by default, when my program starts. I want it to contain a set of functions that are executed only when they are called explicitly.
In the step 4, it is written that I need to "Compile main.cpp and add the library from step 2", but I have already compiled the library on the step 2. Should I compile it one more time?
What does "in this case 'myfile.o'" mean? What exactly should I do with "myfile.o" in this case?
On the step 4 it is written "If you don't add this library, you'll get a error message". But what does "to add library" mean? On step 3 I put #include "mylibrary.h" into the main.cpp, so I have added the library? Or I need to do something more to add the library?
Could you please help me to understand the answer?
ADDED: I think I need to use Eclipse to generate .a, .lib, .so, or .dll files? Does anyone know how I can do it and what exactly I should generate?

Answer 1

For this example, consider a "library" which provides the factorial function. For simplicity, I use free functions.
// factorial.h
int factorial(int input);
// factorial.cpp
#include "factorial.h"
int factorial(int input) {
if(input > 0) return input * factorial(input - 1);
else if(input == 0) return 1;
else return 0;
}
Now compile these files to generate a factorial.o, but do not link. (Configure the Eclipse build command accordingly. If using gcc, use the -c and -o flags appropriately. Refer compiler documentation to understand what these flags do.)
Next create a main.cpp (you can actually name it anything you like.)
// main.cpp
#include "factorial.h"
int main() {
// use your library code here to make sure it is usable and working.
factorial(3);
factorial(5);
// etc.
}
Now compile this to a main.o, then link this together with the factorial.o from earlier to generate the executable. You can actually skip the intermediate main.o step, and do both of this in one step with the appropriate compiler options.