cpp-templates

C++ Templates

Purpose

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.

Triggers

• "How do I read this massive C++ template error?"
• "How do I use concepts to constrain a template?"
• "What's the difference between SFINAE and concepts?"
• "My templates make compilation very slow"
• "How do I write a requires-clause?"
• "How do I profile template instantiation times?"

Workflow

1. Reading template error messages

Template errors print full instantiation chains. Strategy: read from the bottom up.

prog.cpp:25:5: error: no matching function for call to 'sort'
  std::sort(v.begin(), v.end());
  ^~~~~~~~~
/usr/include/c++/13/bits/stl_algo.h:4869:5: note: candidate:
    template<class _RAIter>
    void std::sort(_RAIter, _RAIter)
note: template argument deduction/substitution failed:
prog.cpp:25:5: note: 'MyType' is not a valid type for this template
                             ^~~~~~~~

Rules for reading:

1. Find the first error line (top of output) — that's your code
2. Skip all the note: lines until you find "required from here" or "in instantiation of"
3. The bottom of the stack shows the type that failed substitution

# Limit backtrace depth to reduce noise
g++   -ftemplate-backtrace-limit=3  prog.cpp
clang -ftemplate-depth=32           prog.cpp   # default 1024

# Show simplified errors (GCC 12+)
g++ -fconcepts-diagnostics-depth=3  prog.cpp   # for concept failures

2. SFINAE — legacy constraint technique

SFINAE (Substitution Failure Is Not An Error) silently removes overloads that fail substitution:

#include <type_traits>

// Enable function only for arithmetic types
template <typename T,
    std::enable_if_t<std::is_arithmetic_v<T>, int> = 0>
T square(T x) { return x * x; }

// SFINAE with return type
template <typename T>
auto to_string(T x) -> std::enable_if_t<std::is_integral_v<T>, std::string> {
    return std::to_string(x);
}

// Void-t technique for detecting member existence
template <typename, typename = void>
struct has_size : std::false_type {};

template <typename T>
struct has_size<T, std::void_t<decltype(std::declval<T>().size())>>
    : std::true_type {};