PSS ID Number: 116239
Article Last Modified on 10/16/2002
The information in this article applies to:
- Microsoft C/C++ Compiler (CL.EXE), when used with:
- Microsoft C/C++ for MS-DOS
- Microsoft Visual C++ 1.0
- Microsoft Visual C++ 1.5
- Microsoft Visual C++, 32-bit Editions 1.0
- Microsoft Visual C++, 32-bit Editions 2.0
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 4.1
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2
- Microsoft Visual C++, 32-bit Enterprise Edition 5.0
- Microsoft Visual C++, 32-bit Enterprise Edition 6.0
- Microsoft Visual C++, 32-bit Professional Edition 4.2
- Microsoft Visual C++, 32-bit Professional Edition 5.0
- Microsoft Visual C++, 32-bit Professional Edition 6.0
- Microsoft Visual C++, 32-bit Learning Edition 6.0
- Microsoft Visual C++ .NET (2002)
This article was previously published under Q116239
SYMPTOMS
Code that makes use of the C++ implicit invocation of a user-defined conversion causes the C/C++ compiler to generate the following error message:
C/C++ 9.00 gives the following level 2 warning and errors from the implicit cast and only the warning from the explicit call:
NOTE: In Visual Studio .NET, the error message is:
RESOLUTION
Explicitly invoke the user-defined conversion, as demonstrated in the sample code in the "MORE INFORMATION" section below, rather then depending on the implicit invocation.
STATUS
Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.
MORE INFORMATION
Suppose a virtual-base class A is defined and classes B and C inherit from class A. Furthermore, a class E inherits from both B and C. The two classes, A and B, each provide a virtual user-defined conversion function to "int". The relationship may be represented by the following directed acyclic graph:
A int () }
^ ^
/ \
/ \
B { int () } C {}
^ ^
\ /
\ /
E { }
The dominance rule requires that when you invoke the virtual user-defined conversion function to int from an instance of an object of type E, the version provided by B should be used. This conversion function may be invoked implicitly by assigning an instance of an object of type E to an int. It may also be invoked explicitly through the use of the "operator int" function.
As demonstrated by the following sample code, the compiler accurately identifies and invokes the correct user-defined conversion function when using the explicit call syntax. However, when attempting to make use of the implicit conversion, the compiler produces an error, citing an ambiguity in its search for the correct user-defined conversion function.
Sample Code
/* Compile options needed: /W3
*/
#include <iostream.h>
class A
{
public:
virtual operator int () { return 2; }
};
class B : virtual public A
{
public:
virtual operator int () { return 3; }
};
class C : virtual public A {};
class E : public B, public C {};
void main(void)
{
int i;
int error=0;
E eObject;
// Both of the invocations below should call B's
// operator int ().
if ((i = eObject) != 3) // Fails to compile
error = 1;
if ((i = eObject.operator int()) != 3) // Compiles OK
error = 1;
if (!error)
cout << "PASSED" << endl;
else
cout << "FAILED" << endl;
}
REFERENCES
The Annotated C++ Reference Manual (ARM), Ellis and Stroustrup, section 10.1.1, "Ambiguities," section 10.2, "Virtual Functions," and section 12.3, "Conversions."
Additional query words: kbVC400bug 8.00 8.00c 9.00
Keywords: kbBug kbCompiler kbCPPonly KB116239
Technology: kbAudDeveloper kbCVCComp kbvc100 kbvc150 kbVC200 kbVC32bitSearch kbVC400 kbVC410 kbVC420 kbVC500 kbVC500Search kbVC600 kbVCNET kbVCsearch kbZNotKeyword3 kbZNotKeyword8
