Doctest Cmake

What I Do

Guide setup and usage of the doctest single-header C++17 testing framework in CMake projects, with tests written directly in source files alongside the code they test.

Setup Instructions

1. Verify doctest header exists

Ensure the doctest header is present (e.g. in the below example, it's at third-party/doctest/doctest.h). This file must already exist in the repository.

2. Create test runner entry point

Create test_main.cpp at project root:

#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include "doctest/doctest.h"

This file provides doctest's main() function for the test executable.

3. Update CMakeLists.txt

Add a test executable target alongside your main target:

# Main application (unchanged)
add_executable(your_app
    main.cpp
    your_source.cpp
)

# Disable doctest in main application
target_compile_definitions(your_app PRIVATE DOCTEST_CONFIG_DISABLE)

# Test executable
add_executable(tests
    test_main.cpp
    your_source.cpp  # Same sources as main app
)

target_include_directories(tests PRIVATE
    ${CMAKE_CURRENT_SOURCE_DIR}
    ${CMAKE_CURRENT_SOURCE_DIR}/third-party
)

Key points:

• Include the same source files in both targets
• Include the directory of where doctest.h is, so #include "doctest/doctest.h" resolves
• Use DOCTEST_CONFIG_DISABLE on the main app to strip out all test code
• No preprocessor guards needed in source files - doctest handles this automatically

Writing Tests

Location

Write tests in the .cpp file where the code is defined (the implementation file), not in headers where code is declared.

Pattern

Add tests at the bottom of source files:

// your_source.cpp

#include "your_source.h"

// Implementation code here
int add(int a, int b) {
    return a + b;
}

// Tests - automatically disabled in main app via DOCTEST_CONFIG_DISABLE
#include "doctest/doctest.h"

TEST_CASE("add function") {
    CHECK(add(2, 3) == 5);
    CHECK(add(-1, 1) == 0);
    CHECK(add(0, 0) == 0);
}

Why this pattern

• Tests live next to the code they test for easy maintenance
• DOCTEST_CONFIG_DISABLE in the main app target strips out all test code automatically
• No preprocessor guards needed in source files
• No separate test directory structure needed

Building and Running

cmake -B build
cmake --build build

./build/your_app   # Run application
./build/tests      # Run tests

When to Use This Skill

• Setting up doctest in a new or existing CMake C++ project
• Adding tests to C++ source files
• Understanding the colocated test pattern with conditional compilation