cpp-modules

Name: cpp-modules
Author: mohitmishra786

283installs19forks

How to install cpp-modules?

mohitmishra786/low-level-dev-skills/cpp-modules

$npx -y skills add mohitmishra786/low-level-dev-skills --skill cpp-modules

Installs into the current project.

Summary

C++20 modules skill for modern C++ projects. Use when working with named modules, module partitions, header units, CMake MODULE_SOURCES, Clang -fmodules-ts, BMI caching issues, or migrating from headers to modules. Activates on queries about C++20 modules, import statements, modu

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SKILL.md

1	# C++20 Modules
2
3	## Purpose
4
5	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.
6
7	## Triggers
8
9	- "How do I write a C++20 module?"
10	- "How do I import a module in CMake?"
11	- "What's the difference between a named module and a header unit?"
12	- "My module gives 'cannot find module' errors"
13	- "How do I use C++20 modules with Clang?"
14	- "How do I migrate from headers to modules?"
15
16	## Workflow
17
18	### 1. Module concepts overview
19
20	```
21	C++20 module kinds:
22	├── Named module interface unit (.cppm / .ixx) — exports declarations
23	├── Module implementation unit (.cpp) — defines module members
24	├── Module partition (.cppm) — internal module subdivision
25	└── Header unit (any header) — import a legacy header as module
26	```
27
28	Named modules are the primary target. Header units are a bridge for legacy code. Avoid Global Module Fragment unless required for macro access.
29
30	### 2. Named module — minimal example
31
32	```cpp
33	// math.cppm — module interface unit
34	export module math; // declares the module name
35
36	export int add(int a, int b) { return a + b; }
37	export double pi = 3.14159;
38
39	// Non-exported (module-private)
40	int internal_helper() { return 42; }
41	```
42
43	```cpp
44	// main.cpp — consumer
45	import math; // import the module
46	#include <iostream> // legacy header (still works)
47
48	int main() {
49	std::cout << add(2, 3) << "\n"; // 5
50	std::cout << pi << "\n";
51	}
52	```
53
54	### 3. Module partitions
55
56	```cpp
57	// math-core.cppm — partition
58	export module math:core; // partition 'core' of module 'math'
59
60	export int add(int a, int b) { return a + b; }
61	```
62
63	```cpp
64	// math.cppm — primary module interface
65	export module math;
66	export import :core; // re-export the partition
67	```
68
69	```cpp
70	// math-impl.cpp — implementation unit (no export)
71	module math; // belongs to 'math' module, not a partition
72	// has access to all math declarations, but exports nothing
73	```
74
75	### 4. Header units — bridging legacy headers
76
77	```cpp
78	// Import a standard library header as a module unit
79	import <iostream>; // header unit (compiler generates BMI)
80	import <vector>;
81
82	// Or import a project header (must be compilable as header unit)
83	import "myheader.h";
84	```
85
86	Header units do NOT provide macros to importers. For macro access, use the Global Module Fragment:
87
88	```cpp
89	module; // Global Module Fragment starts here
90	#include <cassert> // macros like assert() are available
91	export module mymod;
92	// ... rest of module
93	```
94
95	### 5. Building with Clang
96
97	```bash
98	# Compile module interface → produces .pcm (precompiled module)
99	clang++ -std=c++20 --precompile math.cppm -o math.pcm
100
101	# Compile implementation using the .pcm
102	clang++ -std=c++20 -fmodule-file=math=math.pcm -c math.cpp -o math.o
103
104	# Compile consumer
105	clang++ -std=c++20 -fmodule-file=math=math.pcm main.cpp math.o -o prog
106	```
107
108	### 6. Building with GCC
109
110	```bash
111	# GCC ≥11 supports modules (experimental ≥11, better ≥14)
112	# Compile interface unit → produces .gcm in gcm.cache/
113	g++ -std=c++20 -fmodules-ts math.cppm -c -o math.o
114
115	# Compiler auto-discovers .gcm files in gcm.cache/
116	g++ -std=c++20 -fmodules-ts main.cpp math.o -o prog
117	```
118
119	### 7. CMake integration (CMake ≥3.28)
120
121	```cmake
122	cmake_minimum_required(VERSION 3.28)
123	project(myproject LANGUAGES CXX)
124	set(CMAKE_CXX_STANDARD 20)
125
126	add_library(math)
127	target_sources(math
128	PUBLIC
129	FILE_SET CXX_MODULES FILES # module interface units
130	src/math.cppm
131	src/math-core.cppm
132	PRIVATE
133	src/math-impl.cpp # implementation unit
134	)
135
136	add_executable(myapp main.cpp)
137	target_link_libraries(myapp PRIVATE math)
138	```
139
140	```bash
141	# Requires a generator that supports modules (Ninja ≥1.11 or MSBuild)
142	cmake -S . -B build -G Ninja
143	cmake --build build
144	```
145
146	For CMake 3.25–3.27 (experimental):
147
148	```cmake
149	cmake_minimum_required(VERSION 3.25)
150	set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "3c375311-a3c9-4396-a187-3227ef642046")
151	set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP ON)
152	```
153
154	### 8. Common errors
155
156	\| Error \| Cause \| Fix \|
157	\|-------\|-------\|-----\|
158	\| `module 'math' not found` \| BMI not found in search path \| Compile interface unit first; check `-fmodule-file=` flags \|
159	\| `cannot import header in module` \| `#include` inside module pu

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Type

cpp-modules

283installs19forks

View on GitHub

How to install cpp-modules?

mohitmishra786/low-level-dev-skills/cpp-modules

$npx -y skills add mohitmishra786/low-level-dev-skills --skill cpp-modules

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# C++20 Modules
2
3	## Purpose
4
5	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.
6
7	## Triggers
8
9	- "How do I write a C++20 module?"
10	- "How do I import a module in CMake?"
11	- "What's the difference between a named module and a header unit?"
12	- "My module gives 'cannot find module' errors"
13	- "How do I use C++20 modules with Clang?"
14	- "How do I migrate from headers to modules?"
15
16	## Workflow
17
18	### 1. Module concepts overview
19
20	```
21	C++20 module kinds:
22	├── Named module interface unit (.cppm / .ixx) — exports declarations
23	├── Module implementation unit (.cpp) — defines module members
24	├── Module partition (.cppm) — internal module subdivision
25	└── Header unit (any header) — import a legacy header as module
26	```
27
28	Named modules are the primary target. Header units are a bridge for legacy code. Avoid Global Module Fragment unless required for macro access.
29
30	### 2. Named module — minimal example
31
32	```cpp
33	// math.cppm — module interface unit
34	export module math; // declares the module name
35
36	export int add(int a, int b) { return a + b; }
37	export double pi = 3.14159;
38
39	// Non-exported (module-private)
40	int internal_helper() { return 42; }
41	```
42
43	```cpp
44	// main.cpp — consumer
45	import math; // import the module
46	#include <iostream> // legacy header (still works)
47
48	int main() {
49	std::cout << add(2, 3) << "\n"; // 5
50	std::cout << pi << "\n";
51	}
52	```
53
54	### 3. Module partitions
55
56	```cpp
57	// math-core.cppm — partition
58	export module math:core; // partition 'core' of module 'math'
59
60	export int add(int a, int b) { return a + b; }
61	```
62
63	```cpp
64	// math.cppm — primary module interface
65	export module math;
66	export import :core; // re-export the partition
67	```
68
69	```cpp
70	// math-impl.cpp — implementation unit (no export)
71	module math; // belongs to 'math' module, not a partition
72	// has access to all math declarations, but exports nothing
73	```
74
75	### 4. Header units — bridging legacy headers
76
77	```cpp
78	// Import a standard library header as a module unit
79	import <iostream>; // header unit (compiler generates BMI)
80	import <vector>;
81
82	// Or import a project header (must be compilable as header unit)
83	import "myheader.h";
84	```
85
86	Header units do NOT provide macros to importers. For macro access, use the Global Module Fragment:
87
88	```cpp
89	module; // Global Module Fragment starts here
90	#include <cassert> // macros like assert() are available
91	export module mymod;
92	// ... rest of module
93	```
94
95	### 5. Building with Clang
96
97	```bash
98	# Compile module interface → produces .pcm (precompiled module)
99	clang++ -std=c++20 --precompile math.cppm -o math.pcm
100
101	# Compile implementation using the .pcm
102	clang++ -std=c++20 -fmodule-file=math=math.pcm -c math.cpp -o math.o
103
104	# Compile consumer
105	clang++ -std=c++20 -fmodule-file=math=math.pcm main.cpp math.o -o prog
106	```
107
108	### 6. Building with GCC
109
110	```bash
111	# GCC ≥11 supports modules (experimental ≥11, better ≥14)
112	# Compile interface unit → produces .gcm in gcm.cache/
113	g++ -std=c++20 -fmodules-ts math.cppm -c -o math.o
114
115	# Compiler auto-discovers .gcm files in gcm.cache/
116	g++ -std=c++20 -fmodules-ts main.cpp math.o -o prog
117	```
118
119	### 7. CMake integration (CMake ≥3.28)
120
121	```cmake
122	cmake_minimum_required(VERSION 3.28)
123	project(myproject LANGUAGES CXX)
124	set(CMAKE_CXX_STANDARD 20)
125
126	add_library(math)
127	target_sources(math
128	PUBLIC
129	FILE_SET CXX_MODULES FILES # module interface units
130	src/math.cppm
131	src/math-core.cppm
132	PRIVATE
133	src/math-impl.cpp # implementation unit
134	)
135
136	add_executable(myapp main.cpp)
137	target_link_libraries(myapp PRIVATE math)
138	```
139
140	```bash
141	# Requires a generator that supports modules (Ninja ≥1.11 or MSBuild)
142	cmake -S . -B build -G Ninja
143	cmake --build build
144	```
145
146	For CMake 3.25–3.27 (experimental):
147
148	```cmake
149	cmake_minimum_required(VERSION 3.25)
150	set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "3c375311-a3c9-4396-a187-3227ef642046")
151	set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP ON)
152	```
153
154	### 8. Common errors
155
156	\| Error \| Cause \| Fix \|
157	\|-------\|-------\|-----\|
158	\| `module 'math' not found` \| BMI not found in search path \| Compile interface unit first; check `-fmodule-file=` flags \|
159	\| `cannot import header in module` \| `#include` inside module pu

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Type