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digitalmars.D.learn - Anyone know why this CTFE isn't working?

reply "Rory McGuire" <rmcguire neonova.co.za> writes:
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import std.stdio;

struct State {
  string s; string getString() { return s; }
  static State opCall(string s) {
  State ret;
  ret.s = s;
  return ret;
  }
}

void main() {
  auto s = State("adf");
  pragma(msg, s.getString());
}

dmd Output: (line 14 is the pragma statement)

struct.d(14): Error: variable s cannot be read at compile time
struct.d(14): Error: cannot evaluate s.getString() at compile time
s.getString()

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<body style="font-family:'DejaVu Sans Mono'; font-size:12px">
<div id="Headers"><div style="padding: 10px 0pt; background-color: rgb(37, 37,
37); color: rgb(255, 255, 255); font-size: 36px; font-family:
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id="Body_content"><div style="padding: 5px; font-family:
Arial,Helvetica,sans-serif; font-size: 12px;"><div><br>import
std.stdio;<br><br>struct State {<br>	string s; string getString() { return s;
}<br>	static State opCall(string s) {<br>		State ret;<br>		ret.s =
s;<br>		return ret;<br>	}<br>}<br><br>void main() {<br>	auto s =
State("adf");<br>	pragma(msg,
s.getString());<br>}<br></div><div><br></div><div>dmd Output: (line 14 is the
pragma statement)</div><div><br>struct.d(14): Error: variable s cannot be read
at compile time<br>struct.d(14): Error: cannot evaluate s.getString() at
compile time<br>s.getString()<br><br><br><br><br></div>
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<strong>Rory McGuire</strong><br>
R&D<br><br>
Tel : +27 (033) 386 7263<br>
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Jul 16 2010
parent reply "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Fri, 16 Jul 2010 11:46:48 +0200, Rory McGuire wrote:


 import std.stdio;
 
 struct State {
   string s; string getString() { return s; } static State opCall(string
   s) {
   State ret;
   ret.s = s;
   return ret;
   }
 }
 
 void main() {
   auto s = State("adf");
   pragma(msg, s.getString());
 }
 
 dmd Output: (line 14 is the pragma statement)
 
 struct.d(14): Error: variable s cannot be read at compile time
 struct.d(14): Error: cannot evaluate s.getString() at compile time
 s.getString()


It's not working because s isn't a compile-time quantity.  Try:

  enum s = State("adf");

-Lars
Jul 16 2010
parent reply "Rory McGuire" <rmcguire neonova.co.za> writes:
On Fri, 16 Jul 2010 11:58:57 +0200, Lars T. Kyllingstad  
<public kyllingen.nospamnet> wrote:


 On Fri, 16 Jul 2010 11:46:48 +0200, Rory McGuire wrote:


 import std.stdio;

 struct State {
   string s; string getString() { return s; } static State opCall(string
   s) {
   State ret;
   ret.s = s;
   return ret;
   }
 }

 void main() {
   auto s = State("adf");
   pragma(msg, s.getString());
 }

 dmd Output: (line 14 is the pragma statement)

 struct.d(14): Error: variable s cannot be read at compile time
 struct.d(14): Error: cannot evaluate s.getString() at compile time
 s.getString()


 It's not working because s isn't a compile-time quantity.  Try:

   enum s = State("adf");

 -Lars


Awesome thanks, worked.

So is the difference that "auto s" is a Struct which can change whereas  
"enum s" is a constant?
If it is a constant its just "s" that is constant right?

Thanks Lars


-Rory
Jul 16 2010
parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Fri, 16 Jul 2010 12:12:38 +0200, Rory McGuire wrote:


 On Fri, 16 Jul 2010 11:58:57 +0200, Lars T. Kyllingstad
 <public kyllingen.nospamnet> wrote:
 

 On Fri, 16 Jul 2010 11:46:48 +0200, Rory McGuire wrote:


 import std.stdio;

 struct State {
   string s; string getString() { return s; } static State
   opCall(string s) {
   State ret;
   ret.s = s;
   return ret;
   }
 }

 void main() {
   auto s = State("adf");
   pragma(msg, s.getString());
 }

 dmd Output: (line 14 is the pragma statement)

 struct.d(14): Error: variable s cannot be read at compile time
 struct.d(14): Error: cannot evaluate s.getString() at compile time
 s.getString()


 It's not working because s isn't a compile-time quantity.  Try:

   enum s = State("adf");

 -Lars


 
 Awesome thanks, worked.
 
 So is the difference that "auto s" is a Struct which can change whereas
 "enum s" is a constant?
 If it is a constant its just "s" that is constant right?
 
 Thanks Lars


Yes.  Writing "auto s = State("adf");" is equivalent to writing

  State s = State("adf");

and since s can change at runtime, it would be meaningless to say that it 
has a value at compile time.  However, its *initial value*, the struct 
literal State("adf"), is known at compile time -- otherwise it would be 
impossible to assign it to an enum.

And actually, an enum is not only a constant, it is a manifest constant.  
When you declare "enum x = someFixedValue", the compiler will just 
replace all later uses of x with someFixedValue.  It's basically the same 
as using the literal value in those places.

This has the consequence that you can't take the address of an enum, nor 
pass an enum by reference.  Doing

  enum i = 3;
  int* p = &i;

is equivalent to

  int* p = &3;

which doesn't make sense.

-Lars
Jul 16 2010