Precompiled Headers with CMake

Using precompiled headers in a CMake project can help reduce compilation times by precompiling commonly used headers. Here’s a step-by-step guide to using precompiled headers in a C++ CMake project:

1. Create a header file with common includes

Create a header file (e.g., pch.h) that includes all commonly used headers in your project. This file will be used as the precompiled header.

// pch.h
#pragma once

#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
// ... other commonly used headers

2. Add a corresponding source file

Create a source file (e.g., pch.cpp) to compile the precompiled header.

// pch.cpp
#include "pch.h"

3. Update your CMakeLists.txt

In your CMakeLists.txt, enable the precompiled headers feature and set the precompiled header file for your project. Also, add the pch.cpp file to your sources.

cmake_minimum_required(VERSION 3.16)  # 3.16 or higher is recommended for precompiled header support
project(MyProject)

set(CMAKE_CXX_STANDARD 14)

# Add your source files
set(SOURCE_FILES main.cpp pch.cpp)

add_executable(MyProject ${SOURCE_FILES})

# Enable precompiled headers
target_precompile_headers(MyProject PRIVATE pch.h)

4. Include the precompiled header in your source files

Now, include the precompiled header (pch.h) at the beginning of each source file in your project.

// main.cpp
#include "pch.h"

int main() {
    // Your code here
}

With these steps, CMake will generate and use precompiled headers for your project, which should help reduce compilation times.

When using precompiled headers, you should include the pch.h file at the beginning of each source file in your project, but you do not need to remove or replace the existing includes.

The idea behind precompiled headers is to speed up the compilation process by precompiling a set of commonly used headers into a single compiled header file. When you include pch.h at the beginning of your source files, the compiler will use the precompiled header file to avoid re-parsing and re-compiling the same headers over and over again.

However, for clarity and maintainability, it’s a good practice to keep the original includes in your source files. This way, other developers can easily understand the dependencies for each source file without having to check the content of the pch.h file.

It’s important to note that if a header file is included in both pch.h and your source file, the compiler will not process the header file twice. It will use the precompiled version from pch.h and effectively ignore the include in your source file.

To summarize, when using precompiled headers:

1. Include the pch.h file at the beginning of each source file in your project.
2. Keep the existing includes in your source files for clarity and maintainability.

Should you distribute precompiled headers?

Distributing precompiled headers is generally not recommended for several reasons:

1. Platform and compiler dependencies: Precompiled headers are specific to the compiler and platform used to create them. Distributing precompiled headers may cause compatibility issues if the user’s platform or compiler differs from the one you used.
2. Binary size: Precompiled headers can be large in size, which could unnecessarily increase the size of your distribution.
3. Difficult to maintain: Since precompiled headers are tied to specific compiler versions and configurations, maintaining them across different platforms and compilers can be challenging.

Instead of distributing precompiled headers, it’s better to provide well-organized source code with proper build system files (such as CMakeLists.txt for CMake) that allow users to compile your project on their own systems. This way, precompiled headers will be generated automatically during the build process on the user’s machine, tailored to their specific environment.

Additionally, you can include instructions on how to use precompiled headers in your project documentation, so users can take advantage of them to speed up their build process if they wish.