Compile time type determination in C++
- by dicroce
A coworker recently showed me some code that he found online. It appears to allow compile time determination of whether a type has an "is a" relationship with another type. I think this is totally awesome, but I have to admit that I'm clueless as to how this actually works. Can anyone explain this to me?
template<typename BaseT, typename DerivedT>
inline bool isRelated(const DerivedT&)
{
DerivedT derived();
char test(const BaseT&); // sizeof(test()) == sizeof(char)
char (&test(...))[2]; // sizeof(test()) == sizeof(char[2])
struct conversion
{
enum { exists = (sizeof(test(derived())) == sizeof(char)) };
};
return conversion::exists;
}
Once this function is defined, you can use it like this:
#include <iostream>
class base {};
class derived : public base {};
class unrelated {};
int main()
{
base b;
derived d;
unrelated u;
if( isRelated<base>( b ) )
std::cout << "b is related to base" << std::endl;
if( isRelated<base>( d ) )
std::cout << "d is related to base" << std::endl;
if( !isRelated<base>( u ) )
std::cout << "u is not related to base" << std::endl;
}
