TITLE: const and storage efficiency


PROBLEM: jskim@rose.etri.re.kr (Kim Jungsun)

In C++, I prefer const to #define as suggested by many gurus.
And I know that "enum" can be used to simulate class specific constants.
My question is then is there any criteria on choosing between "const" and 
"enum" when we want to define constant(s) in gloabl scope? 


RESPONSE: "Guenter Nagler" <gnagler@ihm.tu-graz.ac.at>

As you already guessed there are some advantages of enum constants:
 ... [ good observations deleted ]

e) enum constants cannot be changed at runtime 
   (no storage needed for the constants itself (only enum variables 
   need storage)).


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 8 Jul 95

Const variables of simple type do not necessarily require storage.
That is, the code that is generated using
	enum { size = 1000 };
will probably remain identical if you change the definition to
	const int size = 1000;

On the other hand, const integers are lvalues, so you can take
the address. An enum is not an lvalue, so you can't. Example:

const int iSize = 100;
enum eee { eSize } = 100;

int aray1[iSize]; // ok
int aray2[eSize]; // ok
const int* ip1 = &iSize; // ok, compiler allocates storage for ip1
const eee* ip2 = &eSize; // error, eSize does not have a location


RESPONSE: "Guenter Nagler" <gnagler@ihm.tu-graz.ac.at>

In C/C++ it is allowed to change const values at
runtime indirect by using pointers to them.


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage)

Not so. You can attempt to change a const variable by using casts,
but the results of doing so are undefined. There is no guarantee
that the program will work. In particular, the compiler is allowed
to put the variable in read-only memory.