If substituting template arguments makes the signature ill-formed, that candidate is dropped—not necessarily an error unless no candidate remains.

Are concepts obsolete for SFINAE?

New code often prefers `requires`, but SFINAE remains widespread in libraries and pre-C++20 code.

Hard error instead of SFINAE?

Ill-formedness in contexts where SFINAE does not apply (e.g. certain body errors) still fails compilation—wrap checks with `void_t` / `declval` / concepts.

C++ SFINAE | Substitution Failure Is Not An Error

2026년 3월 12일 · 14분 읽기 · 수정 2026년 3월 12일 Advanced Tutorial

이 글의 핵심

SFINAE lets invalid template substitutions discard overloads instead of hard errors—foundation of many traits and constraints before concepts.

What is SFINAE?

SFINAE: during overload resolution, substitution failure in a template’s immediate context is not an error—it simply removes that candidate.

template<typename T>
std::enable_if_t<std::is_integral_v<T>, T>
process(T v) { return v * 2; }

template<typename T>
std::enable_if_t<std::is_floating_point_v<T>, T>
process(T v) { return v / 2.0; }

Mermaid: resolution flow

flowchart TD
    A[Template call] --> B{Substitution}
    B -->|OK| C[Instantiate]
    B -->|Fail| D{Other overload?}
    D -->|Yes| B
    D -->|No| E[Hard error]
    C --> F[Success]

`std::enable_if`

template<typename T>
std::enable_if_t<std::is_integral_v<T>, void> print(T v);

Expression SFINAE

template<typename T>
auto f(T t) -> decltype(t.size(), void()) { /* ... */ }

SFINAE vs concepts

Concepts (C++20) often express the same constraints more readably:

template<std::integral T>
void func(T v);

enable_if
Type traits

Keywords

C++, SFINAE, enable_if, void_t, templates