Question

On page 340 of the C++ Programming Language: Special Edition, Stroustrup writes…

The semantic checking of a default argument for a template parameter is done if and (only) when that default argument is actually used. In particular, as long as we refrain from using the default template argument Cmp<T> we can compare() strings of a type for which Cmp<X> wouldn't compile (say, because < wasn't defined for an X). This point is crucial in the design of the standard containers, which rely on a template argument to specify default values.

I’m having trouble wrapping my head around the usage of this. Why would this rule allow strings of type X to be compared, when normally it wouldn’t compile? Wouldn’t this behavior be undesirable?

Answer 1

The given example is:

template<class T, class C = Cmp<T> >
int compare(const String<T>& str1, const String<T>& str2) 
{
    // ... compare using C
}

The idea is that the class template Cmp might not be defined or illegal for some T. In that case, you can pass a custom comparison class template:

compare<char, MyComparer>(str1, str2);

If you do that, Cmp isn’t used and won’t be checked if it actually would compile.