[Qt-interest] Weird compiler error with QString(QChar(value))

[Thiago Macieira]

Em Sexta-feira 07 Maio 2010, às 11:19:36, Srdjan Todorovic escreveu:
> Hi,
> 
> On 6 May 2010 19:39, K. Frank <kfrank29.c at gmail.com> wrote:
> > There is a good explanation for this in Marshall Cline's "C++ FAQ LITE":
> >   http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.19
> 
> I just used that as a template for my own test snippet.
> 
> Questions, no doubt off-topic for Qt:
> 
>  1) Why does the C++ compiler / standard allow function declarations
> inside a function?
>      (as opposed to the C-style declarations outside functions that I
> have encountered before)

Heritage from C, I guess.

You can declare external functions inside a function, which means they are 
available in that context only. So:

void foo()
{
    extern void bar();
    bar();
}

void baz()
{
    bar();
}

Will generate an error in baz, because bar is not declared. That seems to be 
the only reason for placing it inside the function.

>       Surely this is not related to function pointers. Or is it?

No.

>       Also does a function declaration inside a function go out of
> scope after the function
>       hits the ret asm instruction? Does it ever go in scope if the
> function is *never* called?

See above.

>       Note that definitions are not allowed (in GCC) and that seems to
> make sense.
> 
>  2) Does anyone ever use this feature of C++?

You can find them in a couple of places in Qt code. It usually happens that you 
need a function from another source file, so you declare the reference to it at 
the point you need.

Also note the "extern" keyword in my example: it's a no-op in that context. 
The "extern" keyword is one of the three mostly-useless keywords in C++98, but 
unlike the other two, it does have an existing meaning. Outside a function, 
these declarations are not the same in C++:

int i;
extern int i;

The first declares a variable called 'i' and exports that symbol; the second is 
simply using the variable 'i' from another context. Interestingly, they are 
the same in C, due to "common symbols".

Exercise for the reader: what are the other two keywords that are mostly 
useless in both C++98 and C99?

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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