Question

I seem to remember I way to select value-pass or reference-pass of a parameter using the size of the type.

Something like:

void fun( check<A> a ){
    ...
}

Generates or:

void fun( A a ){
    ...
}

or

void fun( A & a ){
    ...
}

Depending of the size of type A and the architecture where you compile the applicacion.

Answer 1

In C++11 you can use std::conditional:

#include <type_traits>

class A { ... };

typedef std::conditional<
   std::is_trivially_copyable<A>::value && sizeof(A) <= sizeof(int),
   A, const A&>::type AParam;

// Direct usage
void f(AParam param);

// Wrap into template class
template <typename T> struct check:
   std::conditional<std::is_arithmetic<T>::value, T, const T&> {};

void f(check<A>::type param);

For C++03 compilers you could use Boost implementation – Boost.TypeTraits library.

As @sehe mentioned there is also Boost.CallTraits library that correctly implements required functionality:

#include <boost/call_traits.hpp>

class A { ... };

void f(boost::call_traits<A>::param_type param);