Microsoft Specific

A Microsoft 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);

For information on managed interfaces, see interface class.

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;

__interface implies the novtable __declspec modifier.


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;

        m_i = 0;
        m_bstr = 0;

        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_s("p->int_data = %d\n", p->int_data);
    p->bstr_data = bstr;
    printf_s("bstr_data = %S\n", p->bstr_data);
p->int_data = 100
bstr_data = Testing

END Microsoft Specific

See also

Interface Attributes