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2_CMake_MultiFiles_Libarary.md

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Create a library

This practice is to show

  • how to write a CMakeLists when there are multiple cpp source files
  • how to build a library when we do not want to rebuild wheels

Table of Contents

how to write CMakeLists for multiple files

We already know how to generate an executbale with main.cpp. Now imagine we have a source file, function1.cpp, to implement a fucntion adder. This function is called in our main function.

So, we should write our CamkeLists such that linker can find this function for our main object.

   add_executable(out main.cpp function1.cpp)

This command builds two objects main.cpp.o and function1.cpp.0.

what is a library

A library is a package of code for being reused. Or we can say library free us from rebuilding wheels.

In C++, a library is composed of two parts:

  • .hpp file - a header file that defines the functionality the library is exposing (offering) to the programs using it.
  • .a or .so - a precompiled binary that contains the implementation of that functionality pre-compiled into machine language.

static library VS dynamic library

C++ provide us choices depends on our needs.

The question is: do we need to take wheels with us all the time or just take it when we need it.

That leads to two syles of libaries:

  1. A static library (also known as an archive)

    • compiled and linked directly into your program. When you compile a program that uses a static library, all the functionality of the static library that your program uses becomes part of your executable.
    • .lib for Windows and .a for Linux

  2. A dynamic library (also called a shared library)

    • loaded into your application at run time.
    • .dll for Windows and .so for Linux

  3. Example

    1. build a static lib image info

    2. build a dynamic lib image info

As we can see in the image above, we build a library called libadder.so and this library is linked to our target. However, the static library directly becomes a part of our target instead of an independent existence.