In article <4h775a$m4s@ugress.uib.no> boukanov@afrodite.fi.uib.no (Igor
Boukanov) writes:

|>    My proposal is to change definition of the
|> "integral constant-expression"[5.19] to something like this:

|> An    integral   constant-expression is an expression that can be calculated
|> at the compilation phase.

|> And according to this one would be able to use inline
|> functions in constant-expressions and instead of writing:

|> #define ALIGN(number, base) ((number + base - 1) / base * base)
|> #define MAX(x, y) ((x > y) ? (x) : (y))
|> #define GRAD_TO_RADIAN(x) (x * 3.1415926 / 180)

|>    char buf[ALIGN(50, sizeof(float))];
|>    char buf2[MAX(sizeof(long), sizeof(double))];
|>    const double angle = GRAD_TO_RADIAN(30);

|> one can write:

|> inline unsigned T align(unsigned number, unsigned base) {
|>    return ((number + base - 1) / base * base);
|> }

|> template<class T> inline T max(T x, T y){
|>    return (x > y) ? x : y;

|> inline double grad_to_radian(double x){
|>    return x * 3.1415926 / 180;
|> }

|>    char buf[align(50, sizeof(float))];
|>    char buf2[max(sizeof(long), sizeof(double))];
|>    const double angle = grad_to_radian(30);

|> So what do you think about this?

That it would be nice, but that there are two very big problems:

1. It would basically require a complete C++ interpreter in the
compiler, and

2. It would make the notion of constant expression somewhat
implementation dependant.  Suppose my compiler implements the function
sin as an inline invocation of a compiler built-in.  Can I then use sin
in a constant expression?
--
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
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Igor Boukanov (boukanov@afrodite.fi.uib.no) wrote:
:    My proposal is to change definition of the
: "integral constant-expression"[5.19] to something like this:
:
: An    integral   constant-expression is an expression that can be calculated
: at the compilation phase.
[...]
: So what do you think about this?

With appropriate restrictions on which in-line functions are allowed
in these expressions, it could be very convenient.  Note, however,
that compile-time calculability, like calculability in general, is
recursively undecidable, when you allow functions with recursion or
unbounded loops.

Tom Payne (thp@cs.ucr.edu)
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In article <4hh34b$21r@ugress.uib.no> boukanov@kvark.fi.uib.no (Igor
Boukanov) writes:

|> J. Kanze (kanze@gabi-soft.fr) wrote:
|> > That it would be nice, but that there are two very big problems:

|> > 1. It would basically require a complete C++ interpreter in the
|> > compiler, and

|> But in almost all cases such interpreter is already present in C++
|> compilers! Because of cause all such const look expressions are correct
|> C++ expressions so every C++ compiler has to generate a code that will
|> evaluate them at run time and if forget about the crosscompilation case
|> I think instead of putting the code to object file compiler can just execute
|> it...

Generating code and interpreting it are two different things.  For
example, the code generator only knows about the current scope (stack
frame); the compiler typically has no mechanism for maintaining the
image of multiple stack frames (needed is it is to evaluate a
function) internall.

|> > 2. It would make the notion of constant expression somewhat
|> > implementation dependant.  Suppose my compiler implements the function
|> > sin as an inline invocation of a compiler built-in.  Can I then use sin
|> > in a constant expression?

|> Consider, for example, next:
|> const double x = 1.0 / (very small number);
|> And of cause, the result will depend on exact representation of double...

Agreed.  That is why the above is not a constant expression, according
to the draft standard.  Think about the case of a cross-compiler: it
would be very unintuitive if `1.0 / (very small number)' gave
different results than assigning 1.0 to a double variable, then
dividing the double variable by `(a very small number)'.  A
(cross-)compiler is not required to emulate the floating point
arithmetic of the target machine.

--
James Kanze         Tel.: (+33) 88 14 49 00        email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs-Bourgeois, F-67000 Strasbourg, France
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