cpp-templates

Name: cpp-templates
Author: mohitmishra786

270installs19forks

How to install cpp-templates?

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

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

Installs into the current project.

Summary

C++ template skill for reading template errors and optimizing compile times. Use when deciphering template error stacks, setting -ftemplate-backtrace-limit, writing concepts and requires-clauses, understanding SFINAE vs concepts, or profiling template instantiation bottlenecks wi

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SKILL.md

1	# C++ Templates
2
3	## Purpose
4
5	Guide agents through reading and fixing template error messages, using concepts as cleaner constraints, understanding SFINAE vs concepts trade-offs, and profiling template instantiation depth and compile times with Templight.
6
7	## Triggers
8
9	- "How do I read this massive C++ template error?"
10	- "How do I use concepts to constrain a template?"
11	- "What's the difference between SFINAE and concepts?"
12	- "My templates make compilation very slow"
13	- "How do I write a requires-clause?"
14	- "How do I profile template instantiation times?"
15
16	## Workflow
17
18	### 1. Reading template error messages
19
20	Template errors print full instantiation chains. Strategy: read from the bottom up.
21
22	```text
23	prog.cpp:25:5: error: no matching function for call to 'sort'
24	std::sort(v.begin(), v.end());
25	^~~~~~~~~
26	/usr/include/c++/13/bits/stl_algo.h:4869:5: note: candidate:
27	template<class _RAIter>
28	void std::sort(_RAIter, _RAIter)
29	note: template argument deduction/substitution failed:
30	prog.cpp:25:5: note: 'MyType' is not a valid type for this template
31	^~~~~~~~
32	```
33
34	Rules for reading:
35	1. Find the first error line (top of output) — that's your code
36	2. Skip all the `note:` lines until you find "required from here" or "in instantiation of"
37	3. The bottom of the stack shows the type that failed substitution
38
39	```bash
40	# Limit backtrace depth to reduce noise
41	g++ -ftemplate-backtrace-limit=3 prog.cpp
42	clang -ftemplate-depth=32 prog.cpp # default 1024
43
44	# Show simplified errors (GCC 12+)
45	g++ -fconcepts-diagnostics-depth=3 prog.cpp # for concept failures
46	```
47
48	### 2. SFINAE — legacy constraint technique
49
50	SFINAE (Substitution Failure Is Not An Error) silently removes overloads that fail substitution:
51
52	```cpp
53	#include <type_traits>
54
55	// Enable function only for arithmetic types
56	template <typename T,
57	std::enable_if_t<std::is_arithmetic_v<T>, int> = 0>
58	T square(T x) { return x * x; }
59
60	// SFINAE with return type
61	template <typename T>
62	auto to_string(T x) -> std::enable_if_t<std::is_integral_v<T>, std::string> {
63	return std::to_string(x);
64	}
65
66	// Void-t technique for detecting member existence
67	template <typename, typename = void>
68	struct has_size : std::false_type {};
69
70	template <typename T>
71	struct has_size<T, std::void_t<decltype(std::declval<T>().size())>>
72	: std::true_type {};
73	```
74
75	SFINAE errors are cryptic. Prefer concepts (C++20) for new code.
76
77	### 3. Concepts — modern constraints (C++20)
78
79	```cpp
80	#include <concepts>
81
82	// Define a concept
83	template <typename T>
84	concept Arithmetic = std::is_arithmetic_v<T>;
85
86	template <typename T>
87	concept Printable = requires(T x) {
88	{ std::cout << x } -> std::same_as<std::ostream&>;
89	};
90
91	template <typename T>
92	concept Container = requires(T c) {
93	c.begin();
94	c.end();
95	c.size();
96	typename T::value_type;
97	};
98
99	// Apply concept as constraint
100	template <Arithmetic T>
101	T square(T x) { return x * x; }
102
103	// Abbreviated function template (C++20)
104	auto square(Arithmetic auto x) { return x * x; }
105
106	// requires-clause (more complex conditions)
107	template <typename T>
108	requires Arithmetic<T> && (sizeof(T) >= 4)
109	T big_square(T x) { return x * x; }
110
111	// Concept in auto parameter
112	void print_container(const Container auto& c) {
113	for (const auto& elem : c) std::cout << elem << ' ';
114	}
115	```
116
117	### 4. Requires expressions
118
119	```cpp
120	// requires { expression; } — checks expression is valid
121	// requires { expression -> type; } — checks type of expression
122
123	template <typename T>
124	concept HasPush = requires(T c, typename T::value_type v) {
125	c.push_back(v); // must be valid
126	{ c.front() } -> std::same_as<typename T::value_type&>; // type check
127	{ c.size() } -> std::convertible_to<std::size_t>; // convertible
128	requires std::default_initializable<T>; // nested requirement
129	};
130
131	// Compound requires (all must hold)
132	template <typename T>
133	concept Sortable = requires(T a, T b) {
134	{ a < b } -> std::convertible_to<bool>;
135	{ a == b } -> std::convertible_to<bool>;
136	};
137	```
138
139	### 5. SFINAE vs concepts comparison
140
141	\| Aspect \| SFINAE \| Concepts \|
142	\|--------\|--------\|---------\|
143	\| Syntax \| Complex, verbose \| Clean, readable \|
144	\| Error messages \| Cryptic wall-of-text \| Clear constraint failure \|
145	\| Compile time \| Can be slow (many substitutions) \| Generally faster \|
146	\| C++ version \| C++11 \| C++20 \|
147	\| Short-circuit \| No \| Yes (concept subsumption) \|
148	\| Use in `if constexpr` \| Awkward \| Natural \|
149	\| Overload ranking \| Manually via priority \| Automatic by constrai

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Type

cpp-templates

270installs19forks

View on GitHub

How to install cpp-templates?

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

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

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# C++ Templates
2
3	## Purpose
4
5	Guide agents through reading and fixing template error messages, using concepts as cleaner constraints, understanding SFINAE vs concepts trade-offs, and profiling template instantiation depth and compile times with Templight.
6
7	## Triggers
8
9	- "How do I read this massive C++ template error?"
10	- "How do I use concepts to constrain a template?"
11	- "What's the difference between SFINAE and concepts?"
12	- "My templates make compilation very slow"
13	- "How do I write a requires-clause?"
14	- "How do I profile template instantiation times?"
15
16	## Workflow
17
18	### 1. Reading template error messages
19
20	Template errors print full instantiation chains. Strategy: read from the bottom up.
21
22	```text
23	prog.cpp:25:5: error: no matching function for call to 'sort'
24	std::sort(v.begin(), v.end());
25	^~~~~~~~~
26	/usr/include/c++/13/bits/stl_algo.h:4869:5: note: candidate:
27	template<class _RAIter>
28	void std::sort(_RAIter, _RAIter)
29	note: template argument deduction/substitution failed:
30	prog.cpp:25:5: note: 'MyType' is not a valid type for this template
31	^~~~~~~~
32	```
33
34	Rules for reading:
35	1. Find the first error line (top of output) — that's your code
36	2. Skip all the `note:` lines until you find "required from here" or "in instantiation of"
37	3. The bottom of the stack shows the type that failed substitution
38
39	```bash
40	# Limit backtrace depth to reduce noise
41	g++ -ftemplate-backtrace-limit=3 prog.cpp
42	clang -ftemplate-depth=32 prog.cpp # default 1024
43
44	# Show simplified errors (GCC 12+)
45	g++ -fconcepts-diagnostics-depth=3 prog.cpp # for concept failures
46	```
47
48	### 2. SFINAE — legacy constraint technique
49
50	SFINAE (Substitution Failure Is Not An Error) silently removes overloads that fail substitution:
51
52	```cpp
53	#include <type_traits>
54
55	// Enable function only for arithmetic types
56	template <typename T,
57	std::enable_if_t<std::is_arithmetic_v<T>, int> = 0>
58	T square(T x) { return x * x; }
59
60	// SFINAE with return type
61	template <typename T>
62	auto to_string(T x) -> std::enable_if_t<std::is_integral_v<T>, std::string> {
63	return std::to_string(x);
64	}
65
66	// Void-t technique for detecting member existence
67	template <typename, typename = void>
68	struct has_size : std::false_type {};
69
70	template <typename T>
71	struct has_size<T, std::void_t<decltype(std::declval<T>().size())>>
72	: std::true_type {};
73	```
74
75	SFINAE errors are cryptic. Prefer concepts (C++20) for new code.
76
77	### 3. Concepts — modern constraints (C++20)
78
79	```cpp
80	#include <concepts>
81
82	// Define a concept
83	template <typename T>
84	concept Arithmetic = std::is_arithmetic_v<T>;
85
86	template <typename T>
87	concept Printable = requires(T x) {
88	{ std::cout << x } -> std::same_as<std::ostream&>;
89	};
90
91	template <typename T>
92	concept Container = requires(T c) {
93	c.begin();
94	c.end();
95	c.size();
96	typename T::value_type;
97	};
98
99	// Apply concept as constraint
100	template <Arithmetic T>
101	T square(T x) { return x * x; }
102
103	// Abbreviated function template (C++20)
104	auto square(Arithmetic auto x) { return x * x; }
105
106	// requires-clause (more complex conditions)
107	template <typename T>
108	requires Arithmetic<T> && (sizeof(T) >= 4)
109	T big_square(T x) { return x * x; }
110
111	// Concept in auto parameter
112	void print_container(const Container auto& c) {
113	for (const auto& elem : c) std::cout << elem << ' ';
114	}
115	```
116
117	### 4. Requires expressions
118
119	```cpp
120	// requires { expression; } — checks expression is valid
121	// requires { expression -> type; } — checks type of expression
122
123	template <typename T>
124	concept HasPush = requires(T c, typename T::value_type v) {
125	c.push_back(v); // must be valid
126	{ c.front() } -> std::same_as<typename T::value_type&>; // type check
127	{ c.size() } -> std::convertible_to<std::size_t>; // convertible
128	requires std::default_initializable<T>; // nested requirement
129	};
130
131	// Compound requires (all must hold)
132	template <typename T>
133	concept Sortable = requires(T a, T b) {
134	{ a < b } -> std::convertible_to<bool>;
135	{ a == b } -> std::convertible_to<bool>;
136	};
137	```
138
139	### 5. SFINAE vs concepts comparison
140
141	\| Aspect \| SFINAE \| Concepts \|
142	\|--------\|--------\|---------\|
143	\| Syntax \| Complex, verbose \| Clean, readable \|
144	\| Error messages \| Cryptic wall-of-text \| Clear constraint failure \|
145	\| Compile time \| Can be slow (many substitutions) \| Generally faster \|
146	\| C++ version \| C++11 \| C++20 \|
147	\| Short-circuit \| No \| Yes (concept subsumption) \|
148	\| Use in `if constexpr` \| Awkward \| Natural \|
149	\| Overload ranking \| Manually via priority \| Automatic by constrai

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