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digitalmars.D - Constant initialization

reply Tyro <ridimz_at yahoo.dot.com> writes:
Is there a reason why a const cannot be initialized by the run-time 
return values of a function call? If there is no "good" reason, can we 
have this feature?

void main()
{ const int eint = 5 + 5;        // Legal
   const int rint = 5 + getInt(); // Illegal }

int getInt()
{ return 5; }

Tyro
Dec 09 2004
next sibling parent reply "Simon Buchan" <currently no.where> writes:
On Thu, 09 Dec 2004 04:43:23 -0500, Tyro <ridimz_at yahoo.dot.com> wrote:


 Is there a reason why a const cannot be initialized by the run-time  
 return values of a function call? If there is no "good" reason, can we  
 have this feature?

 void main()
 { const int eint = 5 + 5;        // Legal
    const int rint = 5 + getInt(); // Illegal }

 int getInt()
 { return 5; }

 Tyro


What exactly are you asking here, that the compiler figure out that your  
getInt()
function always returns 5 and swaps it out for a constant 5? Or do you  
want "Semi"
const values: set once at the program start by running that code?

The first is a bit too much to ask, but the effect can be achived with  
fancy templating,
and the second is covered by static constructors. (The documentation is  
your friend here)

-- 
"Unhappy Microsoft customers have a funny way of becoming Linux,
Salesforce.com and Oracle customers." - www.microsoft-watch.com:
"The Year in Review: Microsoft Opens Up"
--
"I plan on at least one critical patch every month, and I haven't been  
disappointed."
- Adam Hansen, manager of security at Sonnenschein Nath & Rosenthal LLP
(Quote from http://www.eweek.com/article2/0,1759,1736104,00.asp)
--
"It's been a challenge to "reteach or retrain" Web users to pay for  
content, said Pizey"
-Wired website: "The Incredible Shrinking Comic"
Dec 09 2004
parent reply Tyro <Tyro_member pathlink.com> writes:
In article <opsiqt2fdrjccy7t simon.mshome.net>, Simon Buchan says...

On Thu, 09 Dec 2004 04:43:23 -0500, Tyro <ridimz_at yahoo.dot.com> wrote:


 Is there a reason why a const cannot be initialized by the run-time  
 return values of a function call? If there is no "good" reason, can we  
 have this feature?

 void main()
 { const int eint = 5 + 5;        // Legal
    const int rint = 5 + getInt(); // Illegal }

 int getInt()
 { return 5; }

 Tyro


What exactly are you asking here, that the compiler figure out that your  
getInt()
function always returns 5 and swaps it out for a constant 5? Or do you  
want "Semi"
const values: set once at the program start by running that code?

The first is a bit too much to ask, but the effect can be achived with  
fancy templating,
and the second is covered by static constructors. (The documentation is  
your friend here)


I want the compler to allow me to use the return value of a function to
initialize a constant variable.

I want to ask the user for input that I can use to initialize a variable who's
value will be constant from that point on. Only way I know to do that is through
the return value of a function.

int main ()
{ const int LENGTH = getLength();
char[LENGTH] myString; }

int getLength()
{ int length;
std.stream.stdin.scanf("%d",&length);
return length; }

If there is some other way to accomplish this, please let me know.

Andrew


"Unhappy Microsoft customers have a funny way of becoming Linux,
Salesforce.com and Oracle customers." - www.microsoft-watch.com:
"The Year in Review: Microsoft Opens Up"
--
"I plan on at least one critical patch every month, and I haven't been  
disappointed."
- Adam Hansen, manager of security at Sonnenschein Nath & Rosenthal LLP
(Quote from http://www.eweek.com/article2/0,1759,1736104,00.asp)
--
"It's been a challenge to "reteach or retrain" Web users to pay for  
content, said Pizey"
-Wired website: "The Incredible Shrinking Comic"
Dec 09 2004
parent "Simon Buchan" <currently no.where> writes:
On Thu, 9 Dec 2004 14:21:10 +0000 (UTC), Tyro <Tyro_member pathlink.com>  
wrote:

<snip>

 I want the compler to allow me to use the return value of a function to
 initialize a constant variable.

 I want to ask the user for input that I can use to initialize a variable  
 who's
 value will be constant from that point on. Only way I know to do that is  
 through
 the return value of a function.

 int main ()
 { const int LENGTH = getLength();
 char[LENGTH] myString; }

 int getLength()
 { int length;
 std.stream.stdin.scanf("%d",&length);
 return length; }

 If there is some other way to accomplish this, please let me know.

 Andrew


 "Unhappy Microsoft customers have a funny way of becoming Linux,
 Salesforce.com and Oracle customers." - www.microsoft-watch.com:
 "The Year in Review: Microsoft Opens Up"
 --
 "I plan on at least one critical patch every month, and I haven't been
 disappointed."
 - Adam Hansen, manager of security at Sonnenschein Nath & Rosenthal LLP
 (Quote from http://www.eweek.com/article2/0,1759,1736104,00.asp)
 --
 "It's been a challenge to "reteach or retrain" Web users to pay for
 content, said Pizey"
 -Wired website: "The Incredible Shrinking Comic"




Not as far as I know, unless you like the idea of doing everything in a
function and passing it that value, like:

main() {
	int input;
	readf(input);
	do_everything(input);
}

void do_everything(const int input) {...}

const is used to determine whether something can be determined at runtime,
though, so your best bet is to just try to not write to it. If you REALLY
want it safe, though, you could wrap in in a class:

class constInt {
	private:
		int _me;
	public:
		this(int init) {
			_me = init;
		}
		int me() {return _me;}
}

(this is where something along the lines of a opCast would be usefull (or
is it already?)

If you are worried about library functions changing it, if they don't NEED
to change it, they can't, if they do, you don't want to use it anyway.

-- 
"Unhappy Microsoft customers have a funny way of becoming Linux,
Salesforce.com and Oracle customers." - www.microsoft-watch.com:
"The Year in Review: Microsoft Opens Up"
--
"I plan on at least one critical patch every month, and I haven't been  
disappointed."
- Adam Hansen, manager of security at Sonnenschein Nath & Rosenthal LLP
(Quote from http://www.eweek.com/article2/0,1759,1736104,00.asp)
--
"It's been a challenge to "reteach or retrain" Web users to pay for  
content, said Pizey"
-Wired website: "The Incredible Shrinking Comic"
Dec 09 2004
prev sibling parent "Garett Bass" <gtbass studiotekne.com> writes:
IIRC, this is how final variables work in Java.  It is 
handy, since you can store data once, and guarantee it will 
not be altered subsequently.

"Tyro" <ridimz_at yahoo.dot.com> wrote in message 
news:cp96o3$2j41$1 digitaldaemon.com...

 Is there a reason why a const cannot be initialized by the 
 run-time return values of a function call? If there is no 
 "good" reason, can we have this feature?

 void main()
 { const int eint = 5 + 5;        // Legal
   const int rint = 5 + getInt(); // Illegal }

 int getInt()
 { return 5; }

 Tyro
Dec 11 2004