CMake Precompiled Header (PCH) Demo

This project demonstrates the impact of Precompiled Headers (PCH) on C++ compile times using CMake.

It builds the same set of source files twice:

1. Without PCH → test_no_pch
2. With PCH → test_with_pch

By comparing build times, you can see how much precompiled headers reduce compilation time in larger C++ projects.

---

Project Structure

.
├── CMakeLists.txt
├── pch.hpp
├── heavy.hpp
├── file1.cpp
├── file2.cpp
├── ...
├── include/
└── README.md

Important Files

• pch.hpp Contains headers that are expensive to compile and are shared across many source files.

• heavy.hpp Simulates a heavy header with many includes.

• file.cpp* Multiple source files that include the heavy headers.

---

Requirements

• CMake 3.20+

• A C++ compiler supporting C++20

  • GCC
  • Clang
  • MSVC

---

Building the Project

Create a build directory and configure the project:

mkdir build
cd build
cmake ..

Build the executables:

cmake --build .

This will generate two binaries:

test_no_pch
test_with_pch

---

Measuring Compile Time

To measure compilation performance, you can use the time command.

Without Precompiled Headers

time cmake --build build --target test_no_pch

With Precompiled Headers

time cmake --build build --target test_with_pch

You should observe faster compile times with PCH, especially when many source files include the same heavy headers.

---

Why Precompiled Headers?

Large C++ projects often include the same headers across hundreds of files.

Examples:

• <vector>
• <string>
• <iostream>
• <algorithm>

Compilers must normally re-parse these headers for every .cpp file.

Precompiled Headers solve this by:

1. Compiling commonly used headers once
2. Reusing the compiled representation for all source files

This can significantly reduce build times in large projects.

---

Example PCH Usage in CMake

CMake provides built-in support:

target_precompile_headers(test_with_pch
    PRIVATE pch.hpp
)

This instructs CMake to generate a precompiled header from pch.hpp and use it for the target.

---

Generating Many Test Files (Optional)

To stress test compilation performance, you can generate many source files:

for i in {1..100}; do
cat <<EOF > file$i.cpp
#include "heavy.hpp"

int func$i()
{
    std::vector<int> v = {1,2,3,4,5};
    return v.size();
}
EOF
done

---

Key Takeaway

Precompiled headers can dramatically reduce compile times when:

• Many translation units include the same headers
• Headers are large or template-heavy
• The project contains hundreds of source files

However, PCH must be managed carefully to avoid unnecessary rebuilds.

---

License

This project is intended for educational and benchmarking purposes.