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John Harrison
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 Posted: Mon Jul 07, 2003 4:46 am Post subject: Re: Why in the world is this ambiguous? |
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"Davis King" <davisking (AT) mail (DOT) com> wrote
| Quote: |
The following code does not compile on any compiler I have access to
(gcc, borland, visual studio). The error I get is that
it can't convert from an int to aaa. But why in the world is this
ambiguous? It's private! Is this a compiler error or is this the
correct behavior? And if so why?
Making the private constructor explicit makes it compile but its not
like a private constructor can be used in this instance in the first
place.
class aaa
{
aaa(int*) { }
public:
aaa(unsigned short a) {}
};
int main()
{
aaa testmeme = 0;
}
-Davis
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Simple. Compiler works out which function to call *before* examining whether
said functions are public or private.
This is as per the standard.
john
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Roger Orr
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| Posted: Thu Jul 10, 2003 3:53 am Post subject: Re: Why in the world is this ambiguous? |
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"Davis King" <davisking (AT) mail (DOT) com> wrote
| Quote: |
The following code does not compile on any compiler I have access to
(gcc, borland, visual studio). The error I get is that
it can't convert from an int to aaa. But why in the world is this
ambiguous? It's private! Is this a compiler error or is this the
correct behavior? And if so why?
|
As others have said, C++ tries to find the correct function first, then sees
if you have access.
| Quote: | Making the private constructor explicit makes it compile but its not
like a private constructor can be used in this instance in the first
place.
|
Making the constructor explicit only works because your code uses
initialisation with '='.
The statement
aaa testmeme = 0;
is valid only if a temporary aaa object can be constructed from 0, and
making a ctor explicit removes it from the possible set of ctors.
[Almost all compilers will optimise away this temporary -- and are
explicitly allowed to -- but the code is only valid if the temporary could
have been constructed]
However, if you (or other users of your class!) use direct initialisation:
aaa testmeme( 0 );
then the code remains ambiguous even when you add explicit to (either)
constructor.
You can resolve the ambiguity by specifying the exact data type:
aaa testmeme( (int*)0 );
but this is a bit messy.
Why do you need both constructors? Perhaps there's another solution to the
problem.
Regards,
Roger Orr
--
MVP in C++ at www.brainbench.com
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