C++20 Modules

Purpose

Guide agents through authoring, building, and debugging C++20 modules: named modules vs header units, module partitions, CMake integration, compiler-specific flags, and interoperability with legacy headers.

Triggers

"How do I write a C++20 module?"
"How do I import a module in CMake?"
"What's the difference between a named module and a header unit?"
"My module gives 'cannot find module' errors"
"How do I use C++20 modules with Clang?"
"How do I migrate from headers to modules?"

Workflow

1. Module concepts overview

C++20 module kinds:
├── Named module interface unit  (.cppm / .ixx)  — exports declarations
├── Module implementation unit   (.cpp)           — defines module members
├── Module partition             (.cppm)          — internal module subdivision
└── Header unit                  (any header)     — import a legacy header as module

Named modules are the primary target. Header units are a bridge for legacy code. Avoid Global Module Fragment unless required for macro access.

2. Named module — minimal example

// math.cppm — module interface unit
export module math;          // declares the module name

export int add(int a, int b) { return a + b; }
export double pi = 3.14159;

// Non-exported (module-private)
int internal_helper() { return 42; }

// main.cpp — consumer
import math;                 // import the module
#include <iostream>          // legacy header (still works)

int main() {
    std::cout << add(2, 3) << "\n";  // 5
    std::cout << pi << "\n";
}

3. Module partitions

// math-core.cppm — partition
export module math:core;     // partition 'core' of module 'math'

export int add(int a, int b) { return a + b; }

// math.cppm — primary module interface
export module math;
export import :core;         // re-export the partition

// math-impl.cpp — implementation unit (no export)
module math;                 // belongs to 'math' module, not a partition
// has access to all math declarations, but exports nothing

4. Header units — bridging legacy headers

// Import a standard library header as a module unit
import <iostream>;           // header unit (compiler generates BMI)
import <vector>;

// Or import a project header (must be compilable as header unit)
import "myheader.h";

Header units do NOT provide macros to importers. For macro access, use the Global Module Fragment:

module;                      // Global Module Fragment starts here
#include <cassert>           // macros like assert() are available
export module mymod;
// ... rest of module

5. Building with Clang

# Compile module interface → produces .pcm (precompiled module)
clang++ -std=c++20 --precompile math.cppm -o math.pcm

# Compile implementation using the .pcm
clang++ -std=c++20 -fmodule-file=math=math.pcm -c math.cpp -o math.o

# Compile consumer
clang++ -std=c++20 -fmodule-file=math=math.pcm main.cpp math.o -o prog

6. Building with GCC

# GCC ≥11 supports modules (experimental ≥11, better ≥14)
# Compile interface unit → produces .gcm in gcm.cache/
g++ -std=c++20 -fmodules-ts math.cppm -c -o math.o

# Compiler auto-discovers .gcm files in gcm.cache/
g++ -std=c++20 -fmodules-ts main.cpp math.o -o prog

7. CMake integration (CMake ≥3.28)

cmake_minimum_required(VERSION 3.28)
project(myproject LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 20)

add_library(math)
target_sources(math
    PUBLIC
        FILE_SET CXX_MODULES FILES    # module interface units
            src/math.cppm
            src/math-core.cppm
    PRIVATE
        src/math-impl.cpp             # implementation unit
)

add_executable(myapp main.cpp)
target_link_libraries(myapp PRIVATE math)

# Requires a generator that supports modules (Ninja ≥1.11 or MSBuild)
cmake -S . -B build -G Ninja
cmake --build build

For CMake 3.25–3.27 (experimental):

cmake_minimum_required(VERSION 3.25)
set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "3c375311-a3c9-4396-a187-3227ef642046")
set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP ON)

8. Common errors

Error	Cause	Fix
`module 'math' not found`	BMI not found in search path	Compile interface unit first; check `-fmodule-file=` flags
`cannot import header in module`	`#include` inside module purview	Move `#include` to Global Module Fragment or use `import <>`
`redefinition of module 'math'`	Two `.cppm` files declare same module	Only one primary interface per module
`macro not available after import`	Macros don't cross module boundaries	Move macro-dependent code to GMF or use `#include`
`ODR violation`	Same name in multiple partitions	Each name exported from exactly one partition
BMI cache stale	`.pcm`/`.gcm` not rebuilt after change	Clean build or ensure dependency tracking is working

9. Interop with legacy headers

// Wrapping a C library for module use
export module cjson;

module;                       // Global Module Fragment
#include <cjson/cJSON.h>      // C header with macros

export module cjson;          // back to module purview

// Re-export key types (optional)
export using ::cJSON;
export using ::cJSON_Parse;

For CMake module support details, see references/modules-cmake-support.md.

Related skills

Use skills/build-systems/build-acceleration for PCH as a modules alternative
Use skills/compilers/gcc or skills/compilers/clang for compiler-specific module flags
Use skills/build-systems/cmake for CMake project configuration

cpp-modules