Heres what I want to do... I want to be able to assign a value to a variable from another variable once and then prevent it from ever being changed again.. Basically something like.. const int x; int a; //... x = a; and then have x act like a regular const However - I know this can''t be done with const (its not defined an inital value) The only half-way easy solution I can think of for this is to have a class that overloads all the operators and acts like a const that can be set by a variable only once.. Though this seems almost trivial to implement I just wish there was a simpler solution.. My Question - Is there one?

There are no direct support for this in C/C++.

Why would you want to do such a thing?

Well, you could abuse the preprocessor to guard your value.

ex:

#define DECL(type, var) type _ ## var; int __ ## var ## _set = 0;
#define SET(var, value) ( __ ## var ## _set == 0 && ( _ ## var = value, __ ## var ## _set++))
#define GET(var) ((const)(_ ## var))

Then in your code:
DECL(int, x);
int a;
SET(x, a);
a *= GET(x);
GET(x) = a; // fails because ((const)(_x)) isn''t a valid lvalue.
SET(x, a); // no-op because _x already has a value assigned.

I think you could try:

const int x;

int a;
int& y = const_cast<int>( x );

y = a;

but it's evil...


Edited by - SteveC on 2/17/00 10:29:23 AM

SteveC is on the right path of doing it. This is how it''s declared withing ANSI C++ standards. In theory you don''t want to change a contants, but sometimes you need too, and this is why the constant cast is within the language

...
const int a = 0;
// a = 0 and contant
a = const_cast(5);
// a = 5 and still constant
...

ZaneX

Isn''t there any support for cloning variables?

ZaneX:

Does that really work?

where you placed the const cast fixes the const-ness but ''int a'' is still an l-value, and a const one at that.
And aren''t you not supposed to be able to assign to a const l-value ?

I didn''t actually try compiling what I thought of, so it could be wrong too

forgot something... It should be like this...

...
int a = conts_cast(5);
...

Now it should work fine.

Actually, What I wrote was fine, Stupid html tags... let''s see if I can get it this time...

a = const_cast<>(5);

...between the <<>> is suppose to be an ''int'' ... whatever, just look in a book or the MSDN help files under const_cast and it will give you the proper syntaxe, but the way I wrote it should work, unless I forgot something...