How are C++ casts implemented?

Question

The C++ casts static_cast, const_cast, reinterpret_cast have a template-like syntax, e.g.

long foo = 3; 
int bar = static_cast<int>(foo);

I've looked in the Standard, and it says that casts are expressions, not template functions as I thought.

This left me wondering: under the hood, are these casts just templates with privileged status, or are they keywords that happen to borrow the template syntax?

Answer 1

are they keywords that happen to borrow the template syntax?

This. Casts are implemented differently depending on the context they are used in – in general, they cannot be implemented as functions. For instance, static_cast is sometimes only a compile-time operation, no code is emitted for it. But other times (in particular when invoking constructors, casting in a type hierarchy or converting between layout-incompatible primitive types) it requires a runtime operation.

That said, you can implement your own functions that resemble the standard cast syntax (boost::lexical_cast does that).