SFINAE - cppreference.com (original) (raw)

"Substitution Failure Is Not An Error"

This rule applies during overload resolution of function templates: When substituting the explicitly specified or deduced type for the template parameter fails, the specialization is discarded from the overload set instead of causing a compile error.

This feature is used in template metaprogramming.

Contents

• 1 Explanation
  • 1.1 Type SFINAE
  • 1.2 Expression SFINAE
  • 1.3 SFINAE in partial specializations
• 2 Library support
• 3 Alternatives
• 4 Examples
• 5 Defect reports

[edit] Explanation

Function template parameters are substituted (replaced by template arguments) twice:

• explicitly specified template arguments are substituted before template argument deduction
• deduced arguments and the arguments obtained from the defaults are substituted after template argument deduction

Substitution occurs in

• all types used in the function type (which includes return type and the types of all parameters)
• all types used in the template parameter declarations
• all types used in the template argument list of a partial specialization

A substitution failure is any situation when the type or expression above would be ill-formed (with a required diagnostic), if written using the substituted arguments.

Only the failures in the types and expressions in the immediate context of the function type or its template parameter types or its explicit specifier(since C++20) are SFINAE errors. If the evaluation of a substituted type/expression causes a side-effect such as instantiation of some template specialization, generation of an implicitly-defined member function, etc, errors in those side-effects are treated as hard errors. A lambda expression is not considered part of the immediate context.(since C++20)

Substitution proceeds in lexical order and stops when a failure is encountered.

template struct B { using type = typename A::type; };   template< class T, class U = typename T::type, // SFINAE failure if T has no member type class V = typename B::type> // hard error if B has no member type // (guaranteed to not occur via CWG 1227 because // substitution into the default template argument // of U would fail first) void foo (int);   template typename T::type h(typename B::type);   template auto h(typename B::type) -> typename T::type; // redeclaration   template void h(...) {}   using R = decltype(h(0)); // ill-formed, no diagnostic required

[edit] Type SFINAE

The following type errors are SFINAE errors:

• attempting to create an array of void, array of reference, array of function, array of negative size, array of non-integral size, or array of size zero:

template void div(char()[I % 2 == 0] = nullptr) { // this overload is selected when I is even }   template void div(char()[I % 2 == 1] = nullptr) { // this overload is selected when I is odd }

• attempting to use a type on the left of a scope resolution operator **::** and it is not a class or enumeration:

template int f(typename T::B*);   template int f(T);   int i = f(0); // uses second overload

• attempting to use a member of a type, where

• the type does not contain the specified member

• the specified member is not a type where a type is required

• the specified member is not a template where a template is required

• the specified member is not a non-type where a non-type is required

template struct X {};   template<template class> struct Z {};   template void f(typename T::Y*) {}   template void g(X<T::N>) {}   template void h(Z<T::template TT>) {}   struct A {}; struct B { int Y; }; struct C { typedef int N; }; struct D { typedef int TT; }; struct B1 { typedef int Y; }; struct C1 { static const int N = 0; }; struct D1 { template struct TT {}; };   int main() { // Deduction fails in each of these cases: f(0); // A does not contain a member Y f(0); // The Y member of B is not a type g(0); // The N member of C is not a non-type h(0); // The TT member of D is not a template   // Deduction succeeds in each of these cases: f(0); g(0); h(0); } // todo: needs to demonstrate overload resolution, not just failure

• attempting to create a pointer to reference
• attempting to create a reference to void
• attempting to create pointer to member of T, where T is not a class type:

template class is_class { typedef char yes[1]; typedef char no[2];   template static yes& test(int C::*); // selected if C is a class type   template static no& test(...); // selected otherwise public: static bool const value = sizeof(test(nullptr)) == sizeof(yes); };

• attempting to give an invalid type to a constant template parameter:

template<class T, T> struct S {};   template int f(S<T, T()>*);   struct X {}; int i0 = f(0); // todo: needs to demonstrate overload resolution, not just failure

• attempting to perform an invalid conversion in

• in a template argument expression

• in an expression used in function declaration:

template<class T, T*> int f(int); int i2 = f<int, 1>(0); // can’t conv 1 to int* // todo: needs to demonstrate overload resolution, not just failure

• attempting to create a function type with a parameter of type void
• attempting to create a function type which returns an array type or a function type

[edit] Expression SFINAE

[edit] SFINAE in partial specializations

Deduction and substitution also occur while determining whether a specialization of a class or variable(since C++14) template is generated by some partial specialization or the primary template. A substitution failure is not treated as a hard-error during such determination, but makes the corresponding partial specialization declaration ignored instead, as if in the overload resolution involving function templates.

// primary template handles non-referenceable types: template<class T, class = void> struct reference_traits { using add_lref = T; using add_rref = T; };   // specialization recognizes referenceable types: template struct reference_traits<T, std::void_t<T&>> { using add_lref = T&; using add_rref = T&&; };   template using add_lvalue_reference_t = typename reference_traits::add_lref;   template using add_rvalue_reference_t = typename reference_traits::add_rref;

[edit] Library support

[edit] Alternatives

Where applicable, tag dispatch, if constexpr(since C++17), and concepts (since C++20) are usually preferred over use of SFINAE.

[edit] Examples

A common idiom is to use expression SFINAE on the return type, where the expression uses the comma operator, whose left subexpression is the one that is being examined (cast to void to ensure the user-defined operator comma on the returned type is not selected), and the right subexpression has the type that the function is supposed to return.

#include   // This overload is added to the set of overloads if C is // a class or reference-to-class type and F is a pointer to member function of C template<class C, class F> auto test(C c, F f) -> decltype((void)(c.*f)(), void()) { std::cout << "(1) Class/class reference overload called\n"; }   // This overload is added to the set of overloads if C is a // pointer-to-class type and F is a pointer to member function of C template<class C, class F> auto test(C c, F f) -> decltype((void)((c->*f)()), void()) { std::cout << "(2) Pointer overload called\n"; }   // This overload is always in the set of overloads: ellipsis // parameter has the lowest ranking for overload resolution void test(...) { std::cout << "(3) Catch-all overload called\n"; }   int main() { struct X { void f() {} }; X x; X& rx = x; test(x, &X::f); // (1) test(rx, &X::f); // (1), creates a copy of x test(&x, &X::f); // (2) test(42, 1337); // (3) }

Output:

(1) Class/class reference overload called (1) Class/class reference overload called (2) Pointer overload called (3) Catch-all overload called

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.