We might use the normal preprocessor something like this
#define WIDTH 42
#define HEIGHT 100
#define AREA (WIDTH*HEIGHT)
But hopefully, cooler heads will prevail.
To have "42*100" is inserted in the code whereever AREA is used. But
does an evaluating preprocessor exist that can do some evaluation on
expressions so that AREA becomes 4200?
No, although the preprocessor does evaluate arithmatic expressions in
some contexts (such as, for an #if directive), most of the time it
merely performs textual substitution.
I know that most compilers will figure this out themselves anyway.
And I know that I should use a const instead for this kind of thing.
Then why not use const? Doing so would guarantee a compile-time
evaluation of 42 * 100:
const int kWidth = 42;
const int kHeight = 100;
const int kArea = kWidth * kHeight;
Since the compiler needs a constant value for the symbol "kArea" it has
no choice but to multiply 42 by 100 at compile time.
But in general (for instance with old (C) compilers that don't have
const, or compilers that don't optimize very well), it would be nice to
_know_ that an expression is evaluated before compile-time and runtime.
This is a C++ newsgroup, so it seems odd to be asking the best practice
for languages other than C++.
And if you want to know whether 42 * 100 is evaluated at runtime or at
compile time, then just examine a disassembly of a compiled binary to
find out. Alternately you could ask your compiler maker whether the
compiler performs "constant folding" as an optimization. And if it
doesn't, find a better compiler.
In general, it would be nice to have a simple and clean turing complete
language for compile time evaluation. I guess I can easily make a
simple but powerful preprocessor in Python that can do this, also for
C/C++ code. But maybe it already exist?
I fervently hope that you do have better uses for your time than to
rewrite the C++ preprocessor. Besides, it has been done, only it's
called "C++ template metaprogramming ".
template <int Nstruct Height
{ static const int value = N; };
template <int Nstruct Width
{ static const int value = N; };
template <class H, class W>
struct Area
{
static const int value = H::value * W::value;
};
Area<Height<42> , Width<100::valu e; // 4200
It's possible to perform much more sophisticated (though not
necessarily any less verbose) compile-time calculations with C++
metaprogramming . At least, for those so inclined.
Greg
