The latest version of this topic can be found at __interface.

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.


 __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  
#define _ATL_ATTRIBUTES 1  
#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