Compiler Error C2872 (C++)

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Visual C++ Concepts: Building a C/C++ Program
Compiler Error C2872

Error Message

'symbol' : ambiguous symbol

The compiler cannot determine which symbol you are referring to.

C2872 can occur if a header file includes a using Directive (C++), and a subsequent header file is #include'd and contains a type that is also in the namespace specified in the using directive. Specify a using directive only after all your header files are specified with #include.

For more information about C2872, see http://support.microsoft.com/default.aspx?scid=kb;en-us;316317.

The following sample generates C2872:

// C2872.cpp
namespace A {
   int i;
}

using namespace A;
int i;
int main() {
   ::i++;   // ok
   A::i++;   // ok
   i++;   // C2872 ::i or A::i?
}
Community Content

Dan Konigsbach
'IServiceProvider' : ambiguous symbol
C2872 sometimes complains about IServiceProvided by surprise for people doing mixed managed/unmanaged code. The general good-practice rules will resolve it:

1) Don't put using namespace at global scope (i.e. not within nested { brackets } ) in any header file.

Unfortunately, that eliminates the convenience of putting lines like using namespace System; near the top of your header file, so that you don't have to qualify all of your CLR references. Sorry, but that's the breaks. 

// MyHeader.h
#pragma once

using namespace System;                       // <-- Don't do this!
using namespace other::nice::stuff;           // <-- Don't do this!

The problem is that when a header has using namespace at the top of the file, it stays in effect after the header. It's considered bad practice, and in particular it hits a conflicting declaration of IServiceProvider.

Your alternatives are:

  • Fully qualify your CLR references (so that every String now becomes System::String)
  • Put the using namespace only inside of some scope, e.g.
ref class CNeatClass
{
    using namespace System;
    // Now I don't have to prefix System:: while I'm in my class declaration!
        :
};

// Now that I'm outside of my class declaration, it's back to prefixing System::
2) In your C++ files, put the #includes before the using namespaces, e.g.
// MyWonderful.cpp

#include "stdafx.h"
#include "MyHeader.h"
#include <WonderCollection>

using namespace System;
using namespace other::nice::stuff;