In C++, SFINAE (Substitution Failure Is Not An Error) is a rule that allows a function template specialization to be excluded from the overload set if substituting the function template parameters with the function arguments would result in a compilation error.

Using SFINAE, the compiler can discard function overloads based on the expressions used in the template parameters, rather than causing a compilation error, which leads to more flexible template metaprogramming and generic programming in C++.

Example

Consider the following code:

#include <type_traits>

template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
void foo(T) {}

int main() {
    foo(5);  // Compiles successfully
    foo(5.5);  // Results in a substitution failure and the function specialization is discarded
}

In this example, the std::enable_if condition is used to enable the function specialization for integral types. When foo(5) is called, the substitution succeeds and the function compiles successfully. However, when foo(5.5) is called, the substitution fails, and the function specialization is discarded due to SFINAE.

Benefits of Expression SFINAE

• Provides a mechanism to conditionally enable or disable function template specializations based on expressions.
• Allows for cleaner and more readable code by avoiding the use of complex and hard-to-read enable_if constructs.
• Enables more intuitive template metaprogramming techniques by leveraging template specialization and type traits.

Expression SFINAE is a powerful feature in modern C++ that enables more flexible and concise template metaprogramming by allowing function template specializations to be selectively included or excluded based on the properties of the template arguments.

For more details, please refer to the C++ standard.

Happy coding!