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template constructor question

 
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Buster
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PostPosted: Mon Aug 07, 2006 9:36 pm    Post subject: template constructor question Reply with quote


{ although it is answered in the FAQ 10.19, I'm letting it in since
there are alternative solutions not necessarily involving copy
initialization, like "Orange o((Lemon()));", for example. -mod }

I'm trying to have a templatized constructor, but when
I use it, it thinks I'm declaring a function:

struct Lemon
{
};

struct Orange
{
template <typename T>
Orange(
const T& t)
{
}

void juice()
{
}
};

void func()
{
Orange o(Lemon());

o.juice(); // error, thinks 'o' is a function
}

If I add another parameter to the constructor,
then it doesn't get confused:

Orange(
const T& t,
int)
....
Orange o(Lemon(), 1);

but I'd prefer fixing this correctly. Is there
some way to do it without this hack ?

Thanks

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Alf P. Steinbach
Guest
PostPosted: Tue Aug 08, 2006 12:33 am    Post subject: Re: template constructor question Reply with quote


* Buster:
Quote:
* Victor Bazarov:
{ although it is answered in the FAQ 10.19, I'm letting it in since
there are alternative solutions not necessarily involving copy
initialization, like "Orange o((Lemon()));", for example. -mod }

Uh, yes, Victor, the guidelines say FAQ questions should be rejected,
but I don't think that's been practiced: many postings I've seen would
then have been rejected. And anyway I don't think it should be
practiced too harshly, because the FAQ isn't and can't be 100% perfect:
both you and I have contributed to the FAQ to help Marshall make it more
perfect, but it's an endless quest. So I think that note is OK. ;-)


Quote:
I'm trying to have a templatized constructor,

It doesn't matter that that it's a template constructor, except that as
a template constructor it isn't a copy constructor.

[snip]
Quote:
template <typename T
Orange( const T& t )

[snip]
Quote:
{
Orange o(Lemon());

o.juice(); // error, thinks 'o' is a function
}

Yes, when at all possible, the compiler must treat declarations as
function declarations.

Here's one way around that, the FAQ solution:

Orange o = Orange( Lemon() );

This requires a copy constructor, which you have implicitly.

The way Victor noted, which is the way noted in the standard, IIRC:

Orange o( (Lemon()) );

A variation, which I think can be better because it's more explicit and
doesn't require deep understand of obscure language rules, just Say NO
To Willynilly Heliotropic* Anonymous Temporaries (the "SNOT-WHAT" rule):

Lemon const aLemon;
Orange o( aLemon );

This works because there's no way to interpret it as a function
declaration, since the argument is a value, not a type.


Quote:
If I add another parameter to the constructor,
then it doesn't get confused:

Orange(
const T& t,
int)
...
Orange o(Lemon(), 1);

Oh, but the compiler does get "confused", as you say, or rather,
defaulting to function declaration interpretation, if you /use it in the
same way/, like

Orange o( Lemon(), int() ); // Oops, function declaration.


Quote:
but I'd prefer fixing this correctly. Is there
some way to do it without this hack ?

See above, and note that your "hack" isn't a solution: you just happened
to use the third variation shown above, supplying a value as argument.


*) OK, English is not my native language, and I just couldn't come up
with a word starting with H.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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