Re: How to subscript a CString? (CSimpleStringT::operator[])

"Doug Harrison [MVP]" <>
Thu, 08 Mar 2007 22:19:35 -0600
On Fri, 9 Mar 2007 09:13:14 +0900, "Norman Diamond"
<ndiamond@community.nospam> wrote:

"Tom Walker" <> wrote in message
[Norman Diamond:]

CString s = _T("ab");
short i = 1;
_TCHAR c = s[i];

Error C2666, ambiguous overload.

The compiler sees an ambiguity between these two possibilities:

_TCHAR c = s.operator[](i);
_TCHAR c = s.operator LPCTSTR()[i];

Thank you, though i am short of complete understanding. If i is changed to
an int then it works. So disambiguation occurs before the integral
promotion, and disambiguation fails because the conversion from short to int
ranks equally with the user-defined cast operator? ("user-defined" from the
standard's point of view because operator LPCTSTR comes from MFC not from
the standard.)

I believe this fragment models your example:

struct X
   int operator[](int);
   operator int*();

void f(X& x)
   int y = x[short(1)];

VC8 emits the error:

X>cl -c a.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for

Copyright (C) Microsoft Corporation. All rights reserved.

a.cpp(9) : error C2666: 'X::operator []' : 2 overloads have similar
        a.cpp(3): could be 'int X::operator [](int)'
        or 'built-in C++ operator[(int *, short)'
        while trying to match the argument list '(X, short)'

This is a bug. Both candidate functions require promotion of short to int,
but the operator[] is an exact match for the implicit object parameter,
while the conversion operator requires conversion of it to int*, which is
worse. (Reading the error message literally, it's like the compiler ignores
the user-defined conversion that occurs when it applies operator*, and to
make matters worse, ignores the fact that short is promoted to int, so
there cannot be any such "signature" for the built-in operator.) Because
operator[] provides a better match on one of the arguments and isn't worse
on the other, it should be selected. You might want to report this here:

Post back with the URL, and I'll verify it.

Doug Harrison
Visual C++ MVP

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