On 2005-02-14 12:22:24 -0500, "rasbury"
<dickyrubbish_removethis_@hotmail.com> said:
[color=blue]
> If I were to include in the definition of a string class the following
> operators:
>
> class String {
> public:
> // ...
> operator const char *() const; // conversion to C-style string
> char operator[](size_t i) const; // character indexing
> // ...
> };
>
> I can't use the indexing operator because the compiler doesn't know whether
> I mean to index the String object directly or to convert to a const char *
> and then index on that. It is clear to me, at least in this case, that
> having gone to the trouble of defining an indexing operator for the class,
> it is that function I want to call. I would have thought that this would be
> obvious to any compiler too - only attempt to perform conversions if an
> expression doesn't make sense without them.[/color]
This runs as you would expect:
#include <iostream>
using std::cout;
class String {
public:
// ...
operator const char *() const
{
cout << "operator const char *() const\n";
return "Some string";
}
char operator[](size_t i) const
{
cout << "operator[](size_t i) const\n";
return 'a';
}
// ...
};
int main()
{
String s;
size_t index = 0;
s[index];
return 0;
}
[color=blue]
>
> Could anyone explain to me why C++ doesn't behave like this? I can't think
> of any example where this would be undesirable (although I could believe
> they might exist), and it seems that it would be very useful, particularly
> when interfacing with C code which expects more primitive types.[/color]
The problem is likely the type you are using for the index. In a
statement like:
s[5]
where s is an instance of String, the 5 is of type int, not size_t, so
any interpretation of that statement requires one of the following
conversions:
String -> const char*
int -> size_t
So, in response to your statement:[color=blue]
> only attempt to perform conversions if an expression doesn't make sense
> without them.[/color]
That's exactly what the compiler *is* doing.
--
Clark S. Cox, III
clarkcox3@gmail.com 
