Question

I want to do this:

template<T>
bool AssertThrows() {
  try {
    T; // T is an expression, so evaluate it here
    return false;
  } catch(...) {
    return true;
  }
}

int main() {
  bool throws = !AssertThrows<5 + 2>();
  return !throws;
}

But this does not work: all I get is these compiler errors:

$ clang++ a.cc
a.cc:1:10: error: unknown type name 'T'
template<T>
         ^
a.cc:4:5: error: use of undeclared identifier 'T'
    T; // T is an expression
    ^
2 errors generated.

Is it possible to have an expression as a template parameter?

Answer 1

In this case a function taking another function to simulate pass by name could work if you don’t mind

template<typename T>
bool AssertThrows(T t) {
  try {
    t(); // t contains an expression, so evaluate it here
    return false;
  } catch(...) {
    return true;
  }
}

int main() {
  bool throws = !AssertThrows([]{ 5 + 2; });
  return !throws;
}