Creating a library

This tutorial only convers the most simple way of writing a library.

If you are working in a large project, or wish to re-distribute your library, you may want to read this more in-depth tutorial: How to write a library

Requirements

We assume you have a qiBuild project containing a executable named foo.

You can use qibuild create foo to get such a project.

We are going to write a function called get_answer() that will return an integer.

Since this function may be used by other people, we are going to put it in a library, called answer

The answer library

Add the following files into your project:

// answer.h

///
/// Get the meaning of life
///
int get_answer();

// answer.cpp

#include "answer.h"

int get_answer()
{
  return 42;
}

Then, edit main.cpp to have:

#include <stdio>
#include "answer.h"

int main()
{
  std::cout << "The answer is: " << get_answer() << std::endl;
  return 0;
}

Using the answer library

In order to use our library in the foo executable, we have to:

  • Find the answer.h file : so we need to add some include directories
  • Create a library named answer with answer.h and answer.cpp
  • Link the foo executable with the answer library.

Adding the include directories

Add the following line to the CMakeLists.txt:

include_directories(".")

Note

CMake always interprets paths relative to the current CMakeLists file So since the CMakeLists and your headers are in the same directory, include_directories(”.”) is enough

Creating the answer library

Add a call to qi_create_lib:

qi_create_lib(answer answer.h answer.cpp)

This creates a static library by default, named libanswer.a on UNIX, and answer.lib or answer_d.lib on Windows.

It also makes the answer library usable by other targets.

Building

You can then build your project.

A few notes:

  • On Windows, the library will be found in build/sdk/lib/answer_d.lib if built in debug, or in build/sdk/lib/answer.lib if built in release.
  • On linux, the library will be found in build/sdk/lib/libanswer.so
  • On mac, the library will be fon in build/sdk/lib/libanswer.dylib

Note

On UNIX, you can force the creation of static library by using -DBUILD_SHARED_LIBS=OFF

On Windows, the sources need to be patched to use answer as a shared library, but this out of the scope of this documentation.

Conclusion

The final CMakeLists.txt code looks like

cmake_minimum_required(VERSION 2.6.4)
include(qibuild.cmake)
project(foo)

include_directories(".")
qi_create_lib(answer answer.h answer.cpp)

qi_create_bin(foo main.cpp)
qi_use_lib(foo answer)
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