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Microsoft Specific

A Visual C++ interface can be defined as follows:

  • Can inherit from zero or more base interfaces.
  • Cannot inherit from a base class.
  • Can only contain public, pure virtual methods.
  • Cannot contain constructors, destructors, or operators.
  • Cannot contain static methods.
  • Cannot contain data members; properties are allowed.
modifier __interface interface-name {interface-definition}

A C++ class or struct could be implemented with these rules, but __interface enforces them.

For example, the following is a sample interface definition:

__interface IMyInterface 
   HRESULT CommitX();
   HRESULT get_X(BSTR* pbstrName);

Notice that you do not have to explicitly indicate that the CommitX and get_X functions are pure virtual. An equivalent declaration for the first function would be:

virtual HRESULT CommitX() = 0;

It is possible to define a managed interface. For example,

__gc __interface IMyInterface 


Only managed classes can implement managed interfaces. See __gc for more information.

__interface implies the novtable __declspec modifier.

For more information about interfaces, see 6 gc Interfaces.


The following sample shows how to use properties declared in an interface.

// deriv_interface.cpp
#include <atlbase.h>
#include <atlcom.h>
#include <string.h>
#include <comdef.h>
#include <stdio.h>


[ object, uuid("00000000-0000-0000-0000-000000000001"), library_block ]
__interface IFace {
   [ id(0) ] int int_data;
   [ id(5) ] BSTR bstr_data;

[ coclass, uuid("00000000-0000-0000-0000-000000000002") ]
class MyClass : public IFace {
   int m_i;
   BSTR m_bstr; 

   MyClass() {
      m_i = 0;
      m_bstr = 0;

   ~MyClass() {
      if (m_bstr) 

   int get_int_data() {
      return m_i;

   void put_int_data(int _i) {
      m_i = _i;

   BSTR get_bstr_data() { 
      BSTR bstr = ::SysAllocString(m_bstr);
      return bstr; 

   void put_bstr_data(BSTR bstr) { 
      if (m_bstr) 
      m_bstr = ::SysAllocString(bstr);

int main() {
   _bstr_t bstr("Testing");
   CComObject<MyClass>* p;
   p->int_data = 100;
   printf("p->int_data = %d\n", p->int_data);              
   p->bstr_data = bstr;
   printf("bstr_data = %S\n", p->bstr_data);


p->int_data = 100
bstr_data = Testing

END Microsoft Specific

See Also

C++ Keywords | Interface Attributes