SFINAE is the provides the essential “Patten matching” like feature to C++ template meta-programming.
template <class T> size_t haha(const T &t) { return t.size(); }
template <class T> void func(const T &t) { cout << "func" << endl; }
template <class T> void func(T &&t) { cout << haha(t) << endl; }
int main(int argc, const char *argv[]) {
int i = 0;
func(i);
return 0;
}
Compiler says:
./test.cc:10:13: error: member reference base type 'const int' is not a structure or union
return t.size();
~^~~~~
./test.cc:20:13: note: in instantiation of function template specialization 'haha<int>' requested here
cout << haha(t) << endl;
^
./test.cc:26:5: note: in instantiation of function template specialization 'func<int &>' requested here
func(i);
^
1 error generated.
T && matches int better than const T & as we can see. But the compiler does not try the template <class T> size_t haha(const T &t), since this is not SFINAE!
from cppreference:
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.
There is no SFINAE here, so there is a error. That’s a common mistake when writing c++ template functions for a beginner (Yes, it’s me!).