Re: [boost] Strange gcc 3.2.2-5.8 behavior

From: Jonathan Wakely (cow_at_[hidden])
Date: 2005-02-11 04:24:52

On Fri, Feb 11, 2005 at 02:23:42AM +0000, Tony Han Bao wrote:

> Hi John,
> 
> From the standard:
> 
> 9.4.2 Static data members
> 
> 4 If a static data member is of const integral or const enumeration 
> type, its declaration in the class definition can specify a 
> constant-initializer which shall be an integral constant expression 
> (5.19). In that case, the member can appear in integral constant 
> expressions within its scope. The member shall still be defined in a 
> namespace scope if it is used in the program and the namespace scope 
> definition shall not contain an initializer.
> 
> so adding the line
> 
> 	const short S::l1;
> 
> solves the problem.

Note this is documented here:
http://gcc.gnu.org/onlinedocs/gcc/Static-Definitions.html#Static-Definitions

> But I still don't understand neither why passing by value doesn't cause 
> the error.

Because when passing by value S::l1 is only used as an integral constant
expression, to initialise a variable. Passing by reference is similar to
taking its address, which requires it to *have* and address, which
requires if to have storage, so it must have been defined somewhere.

jon

> On 11 Feb 2005, at 01:03, John Eddy wrote:
> 
> >The following bit of code gives me an undefined symbol (l1) when using 
> >gcc 3.2.2-5.8.  Is there something wrong with the code or is there a 
> >bug in the compiler or a setting I don't know about, etc?
> >
> >
> >struct S { static const short l1 = 10; };
> >
> >void go(const short& l) {}; // causes undefined symbol l1
> >
> >int main(int argc, char* argv[]) {  go(S::l1);  return 0; }
> >
> >
> >If I do normal, non-in-class-initialization, the problem doesn't 
> >appear nor does it appear if I accept the short by value in go.

-- 
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	- C3PO