Compiler Error C2259

'class' : cannot instantiate abstract class

Code declares an instance of an abstract class or structure.

You can't instantiate a class or structure with one or more pure virtual functions. To instantiate objects of a derived class, the derived class must override each pure virtual function.

For more information, see Implicitly abstract classes.

The following sample generates C2259:

// C2259.cpp
// compile with: /c
class V {
public:
   void virtual func() = 0;
};
class A : public V {};
class B : public V {
public:
   void func();
};
V v;  // C2259, V is an abstract class
A a;  // C2259, A inherits func() as pure virtual
B b;  // OK, B defines func()

Whenever you derive from an interface and implement the interface methods in the derived class with access permissions other than public, you may receive C2259. The error occurs because the compiler expects the interface methods implemented in the derived class to have public access.

To resolve this issue, don't use more restrictive access permissions for the implementation methods. The compiler doesn't consider them as implementations for the interface methods defined in the interface. In turn, that makes the derived class an abstract class. Instead, make the access permissions public for the implemented methods.

C2259 can also occur because of conformance work that was done in Visual Studio 2005, /Zc:wchar_t is now on by default. In this situation, C2599 can be resolved either by compiling with /Zc:wchar_t-, to get the behavior from previous versions, or preferably, by updating your types so they're compatible. For more information, see /Zc:wchar_t (wchar_t Is Native Type).

The following sample generates C2259:

// C2259b.cpp
// compile with: /c
#include <windows.h>

class MyClass {
public:
   // WCHAR now typedef'ed to wchar_t
   virtual void func(WCHAR*) = 0;
};

class MyClass2 : MyClass {
public:
   void func(unsigned short*);
};

MyClass2 x;   // C2259

// OK
class MyClass3 {
public:
   virtual void func(WCHAR*) = 0;
   virtual void func2(wchar_t*) = 0;
   virtual void func3(unsigned short*) = 0;
};

class MyClass4 : MyClass3 {
public:
   void func(WCHAR*) {}
   void func2(wchar_t*) {}
   void func3(unsigned short*) {}
};

MyClass4 y;

The following sample generates C2259:

// C2259c.cpp
// compile with: /clr
interface class MyInterface {
   void MyMethod();
};

ref class MyDerivedClass: public MyInterface {
private:
   // Uncomment the following line to resolve.
   // public:
   virtual void MyMethod(){}
};

int main() {
   MyDerivedClass^ instance = gcnew MyDerivedClass; // C2259
}