CMake configure_file with out of source build

Question

I'm trying to use CMake to create some header files that will be read in by my source code. My problem is that when I run cmake .. in my build/ folder, it generates the configuration file inside my build folder, and then when I run the generated Makefile it fails to find the header.

Of course I could fix this by using #include "build/config.h" from my C++ file rather than #include "config.h", but it seems like my C++ code shouldn't know about my build folder (I might want more than one possible build setup, for example). Similarly, I could ask CMake to write the header file to the source directory, but that's breaking the out-of-source build setup.

Is there a way of getting CMake to generate a Makefile that will build, subject to these constraints?

My CMakeLists.txt is below. It reads in config.h.in and outputs config.h.

cmake_minimum_required(VERSION 3.9)
project(fftb)

set(CMAKE_CXX_STANDARD 17)

set(fftb_VERSION_MAJOR 0)
set(fftb_VERSION_MINOR 0)

configure_file(
    "${PROJECT_SOURCE_DIR}/config.h.in"
    "${PROJECT_BINARY_DIR}/config.h"
)

add_executable(fftb main.cpp config.h.in)

My config.h.in is below, it #defines the version to be used in main.cpp

#ifndef FFTB_CONFIG_H_IN_H
#define FFTB_CONFIG_H_IN_H

#define fftb_VERSION_MAJOR @fftb_VERSION_MAJOR@
#define fftb_VERSION_MINOR @fftb_VERSION_MINOR@

#endif //FFTB_CONFIG_H_IN_H

My main.cpp is below, it includes the auto-generated config.h and outputs it.

#include <iostream>
#include "config.h"

int main() {
    std::cout << "Version " << fftb_VERSION_MAJOR << "." << fftb_VERSION_MINOR << std::endl;
    return 0;
}

What if you wrote `add_executable(fftb main.cpp "${PROJECT_BINARY_DIR}/config.h")`? — Justin, Mar 02 '18 at 20:16
Why you don't use include_directories to add the directory containing the .h file ( in this case PROJECT_BINARY_DIR)? — Meysam Mohammadi, Mar 02 '18 at 20:20

score 2 · Accepted Answer · answered Mar 02 '18 at 20:17

2

You could add your build directory to the list of directories to search for include files by adding something like include_directories(${PROJECT_BINARY_DIR}) to your CMakeLists.txt

answered Mar 02 '18 at 20:17

shay

755
10
22

5

Do not use `include_directories`. In *Modern CMake*, you want to use the `target_*` functions instead – Justin Mar 02 '18 at 20:20
3

This (or using cmake target_include_directories) is the correct solution. – Stephen Newell Mar 02 '18 at 20:20

CMake configure_file with out of source build

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