# SFINAE

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"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.

### 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 expressions used in the function type all expressions used in a template parameter declaration | (since C++11)

all expressions used in the explicit specifier | (since C++20)

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)

| This section is incomplete Reason: mini-example where this matters

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

If there are multiple declarations with different lexical orders (e.g. a function template declared with trailing return type, to be substituted after a parameter, and redeclared with ordinary return type that would be substituted before the parameter), and that would cause template instantiations to occur in a different order or not at all, then the program is ill-formed; no diagnostic required. | (since C++11)

    template<typename A>
    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<T>::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<class T>
    typename T::type h(typename B<T>::type);

    template<class T>
    auto h(typename B<T>::type) -> typename T::type; // redeclaration

    template<class T>
    void h(...) {}

    using R = decltype(h<int>(0));     // ill-formed, no diagnostic required

### Type SFINAE

The following type errors are SFINAE errors:

attempting to instantiate a pack expansion containing multiple packs of different lengths | (since C++11)

- 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<int I>
    void div(char(*)[I % 2 == 0] = nullptr)
    {
        // this overload is selected when I is even
    }

    template<int I>
    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<class T>
    int f(typename T::B*);

    template<class T>
    int f(T);

    int i = f<int>(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<int I>
    struct X {};

    template<template<class T> class>
    struct Z {};

    template<class T>
    void f(typename T::Y*) {}

    template<class T>
    void g(X<T::N>*) {}

    template<class T>
    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<typename T>
        struct TT {};
    };

    int main()
    {
        // Deduction fails in each of these cases:
        f<A>(0); // A does not contain a member Y
        f<B>(0); // The Y member of B is not a type
        g<C>(0); // The N member of C is not a non-type
        h<D>(0); // The TT member of D is not a template

        // Deduction succeeds in each of these cases:
        f<B1>(0);
        g<C1>(0);
        h<D1>(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<typename T>
    class is_class
    {
        typedef char yes[1];
        typedef char no[2];

        template<typename C>
        static yes& test(int C::*); // selected if C is a class type

        template<typename C>
        static no& test(...);       // selected otherwise
    public:
        static bool const value = sizeof(test<T>(nullptr)) == sizeof(yes);
    };

- attempting to give an invalid type to a non-type template parameter:

    template<class T, T>
    struct S {};

    template<class T>
    int f(S<T, T()>*);

    struct X {};
    int i0 = f<X>(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

### Expression SFINAE

Only constant expressions that are used in types (such as array bounds) were required to be treated as SFINAE (and not hard errors) before C++11. | (until C++11)
The following expression errors are SFINAE errors Ill-formed expression used in a template parameter type Ill-formed expression used in the function type:
    struct X {};
    struct Y { Y(X){} }; // X is convertible to Y

    template<class T>
    auto f(T t1, T t2) -> decltype(t1 + t2); // overload #1

    X f(Y, Y);                               // overload #2

    X x1, x2;
    X x3 = f(x1, x2); // deduction fails on #1 (expression x1 + x2 is ill-formed)
                      // only #2 is in the overload set, and is called
| (since C++11)

### 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. Compilers do not treat a substitution failure as a hard-error during such determination, but ignore the corresponding partial specialization declaration 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<class T>
    struct reference_traits<T, std::void_t<T&>>
    {
        using add_lref = T&;
        using add_rref = T&&;
    };

    template<class T>
    using add_lvalue_reference_t = typename reference_traits<T>::add_lref;

    template<class T>
    using add_rvalue_reference_t = typename reference_traits<T>::add_rref;

Notes: currently partial specialization SFINAE is not formally supported by the standard (see also CWG issue 2054), however, LFTS requires it works since version 2 (see also detection idiom).

### Library support

The standard library component std::enable_if allows for creating a substitution failure in order to enable or disable particular overloads based on a condition evaluated at compile time. In addition, many type traits must be implemented with SFINAE if appropriate compiler extensions are unavailable. | (since C++11)

The standard library component std::void_t is another utility metafunction that simplifies partial specialization SFINAE applications. | (since C++17)

### Alternatives

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

`static_assert` is usually preferred over SFINAE if only a conditional compile time error is wanted. | (since C++11)

### 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.

Run this code

    #include <iostream>

    // 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

### Defect reports

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

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 295 | C++98 | creating cv-qualified function type could result in substitution failure | made not failure, discarding cv-qualification
CWG 1227 | C++98 | the order of substitution was unspecified | same as the lexical order
CWG 2322 | C++11 | declarations in different lexical orders would cause template instantiations to occur in a different order or not at all | such case is ill-formed, no diagnostic required

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