SFINAE
SFINAE
"Substitution Failure Is Not An Error"
This rule applies during overload resolution of function templates: When substituting the 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
Substitution occurs in.
- all types used in the function type (which includes return type and the types of all parameters)
all expressions used in the function type all expressions used in a template parameter declaration | (since C++11) |
---|
- all expressions used in the function type
(since C++11)
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 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.
Substitution proceeds in lexical order and stops when a failure is encountered. template | (since C++14) |
---|
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 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 abstract class types, array of negative size, or array of size zero.
template<int I> void div(char(*)[I % 2 == 0] = 0) {
// this overload is selected when I is even
}
template<int I> void div(char(*)[I % 2 == 1] = 0) {
// this overload is selected when I is odd
}
- attempting to use a type that is not a class or enumeration on the left of a scope resolution operator
::
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
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
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>(0)) == 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
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 cv-qualified function type | (until C++11) |
---|---|
attempting to create a function type with a parameter or return type that is an abstract class. | (since C++11) |
- attempting to create a cv-qualified function type
(until C++11)
- attempting to create a function type with a parameter or return type that is an abstract class.
(since C++11)
Expression SFINAE
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 | (since C++11) |
---|---|
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) |
- Ill-formed expression used in a template parameter 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) 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)
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.
The standard library component std::void_t
is another utility metafunction that simplifies SFINAE applications.
In addition, many type traits are implemented using SFINAE.
Alternatives
Where applicable, tag dispatch, static_assert, and, if available, concepts, are usually preferred over direct use of SFINAE.
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 <iostream>
// this overload is always in the set of overloads
// ellipsis parameter has the lowest ranking for overload resolution
void test(...)
{
std::cout << "Catch-all overload called\n";
}
// this overload is added to the set of overloads if
// C is a reference-to-class type and F is a member function pointer
template<class C, class F>
auto test(C c, F f) -> decltype((void)(c.*f)(), void())
{
std::cout << "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 member function pointer
template<class C, class F>
auto test(C c, F f) -> decltype((void)((c->*f)()), void())
{
std::cout << "Pointer overload called\n";
}
struct X { void f() {} };
int main(){
X x;
test( x, &X::f
test(&x, &X::f
test(42, 1337
}
Output:
Reference overload called
Pointer overload called
Catch-all overload called
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