01-configure-files

01-configure-files

Table of Contents

• Introduction
• Concepts
  • Configure Files
• Building the Example

Introduction

During the call to cmake it is possible to create files that use variables from the CMakeLists.txt and cmake cache. During CMake generation the file is copied to a new location and any cmake variables are replaced.

The files in this tutorial are below:

01-configure-files$ tree
.
├── CMakeLists.txt
├── main.cpp
├── path.hpp.in
├── version.hpp.in

• CMakeLists.txt - Contains the CMake commands you wish to run

• main.cpp - The source file with main

• path.hpp.in - File to contain a path to the build directory

• version.hpp.in - File to contain the version of the project

Concepts

Configure Files

To do variable substitution in a file you can use the configure_file() function in CMake. This core arguments for this function are source file and destination file.

configure_file(version.hpp.in ${PROJECT_BINARY_DIR}/version.hpp)

configure_file(path.hpp.in ${PROJECT_BINARY_DIR}/path.hpp @ONLY)

The first example above, allows the variable to be defined like a CMake variables using the ${} syntax or an @@ in the version.hpp.in file. After generation a new file version.hpp will be available in the PROJECT_BINARY_DIR.

namespace my
{
	// version variable that will be substituted by cmake
	// This shows an example using the $ variable type
	const char* project_version = "${${PROJECT_NAME}_VERSION}";
}

The second example, only allows variables to be defined using the @@ syntax in the path.hpp.in file. After generation a new file path.hpp will be available in the PROJECT_BINARY_DIR.

namespace my
{
	// version variable that will be substituted by cmake
	// This shows an example using the @ variable type
	const char* project_path = "@CMAKE_SOURCE_DIR@";
}

Building the Example

$ mkdir build

$ cd build/

$ cmake ..
-- The C compiler identification is ...
-- The CXX compiler identification is ...
-- Check for working C compiler: ...
-- Check for working C compiler: ... -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: ...
-- Check for working CXX compiler: ... -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: project-absolute-path/01-configure-files/build

$ ls
CMakeCache.txt  CMakeFiles  cmake_install.cmake  Makefile  path.hpp  ver.hpp

$ cat path.hpp
#pragma once

namespace my
{
	// version variable that will be substituted by cmake
	// This shows an example using the @ variable type
	const char* project_path = "project-absolute-path/01-configure-files";
}

$ cat version.hpp
#pragma once

namespace my
{
	// version variable that will be substituted by cmake
	// This shows an example using the $ variable type
	const char* project_version = "1.2.3";
}