FIX: Compiler Error C2784 o
ID: Q166721
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The information in this article applies to:
-
The C/C++ Compiler (CL.EXE), included with:
-
Microsoft Visual C++, 32-bit Editions, version 5.0
SYMPTOMS
When you attempt to instantiate an STL container such as vector or list,
you receive compiler error C2784 if the class type of the objects contained
do not have operator < and operator == defined to take the class type as
both arguments:
xutility(45) : error C2784: 'bool __cdecl std::operator<(const class
std::reverse_iterator<`template-parameter-1',`template-parameter-2
',`template-parameter-3',`template-parameter-4',`template-parameter-5'>
&,const class std::reverse_iterator<`template-parameter-1', `template-
parameter-2',`template-parameter-3',`template-parameter-4',
`template-parameter-5'> &)' : could not deduce template argument for
'const class std::reverse_iterator<`template-parameter-1',
`template-parameter-2',`template-parameter-3',`template-parameter-4',
`template-parameter-5'> &' from 'const struct P'
xutility(45) : error C2676: binary '<' : 'const struct MyClass' does not
define this operator or a conversion to a type acceptable to the
predefined operator
CAUSE
The Visual C++ 5.0 compiler forces the instantiation of inline template
member functions regardless of whether they are referenced. The member
function generated is later discarded if it is not referenced.
RESOLUTION
Declare a global operator < and a global operator == that take the given
class type in both arguments. Note that the operators don't need to have
bodies because they aren't actually used and will be discarded by the
linker anyway.
#if (_MSC_VER == 1100)
#define GLOBAL_OPS(Type) \
bool operator < (const Type& lhs, const Type& rhs);
bool operator == (const Type& lhs, const Type& rhs);
GLOBAL_OPS(MyClass)
GLOBAL_OPS(MyOtherClass)
#endif
STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed
at the beginning of this article. This bug has been corrected in Visual
Studio 97 Service Pack 1.
For additional information about the Visual Studio 97 Service Pack 1,
please see the following article in the Microsoft Knowledge Base:
Q170365
INFO: Visual Studio 97 Service Packs - What, Where, and Why
MORE INFORMATION
Note that if you use functions such as find() or sort() or compare
containers (using one of the logical operators), you must define less than
and equality operators for the class type being contained in the STL
container.
Steps to Reproduce Behavior
/*
Compile options needed to reproduce: -GX
Compile options needed for workaround: -GX -D"WORKAROUND"
*/
#include <vector>
using namespace std;
struct MyClass {
// members here
};
struct MyOtherClass {
// members here
};
#ifdef WORKAROUND
#if (_MSC_VER == 1100)
#define GLOBAL_OPS(Type) \
bool operator < (const Type& lhs, const Type& rhs) {return true;} \
bool operator == (const Type& lhs, const Type& rhs) {return true;}
GLOBAL_OPS(MyClass)
GLOBAL_OPS(MyOtherClass)
#endif //_MSC_VER
#endif //WORKAROUND
void main () {
vector<MyClass> a;
}
Keywords : kbcode kbtool kbCompiler kbCPPonly kbLangCPP kbVC kbVC500bug kbVS97sp1fix STLIss
Version : 5.0
Platform : NT WINDOWS
Issue type : kbbug