C++ pure virtual functions implementation in derived class

Question

I have a top API (based on abstract class) which is composed by a base API (that will be reused by different top APIs).

class api_base {
public:
    virtual int foo() = 0;
};

class api_top : public api_base {
public:
    virtual int bar() = 0;
}

Then, I want to provide a base implementation for base API:

class imp_base {
public:
    int foo() { return 1; }
}

And finally implement top API using base implementation for those functions defined in base API:

class imp_top : public api_top, public imp_base {
public:
    int bar() { return 1; }
}

When I instantiate an object of imp_top type, compiler say that foo() function is not implemented. But that is not true, since imp_top derives from imp_base, which do have foo() function implemented.

Any advice about this?

Thank you in advance.

Full test code:

    #include <stdio.h>

    class api_base { 
    public:
        virtual int foo() = 0;
    };

    class api_top : public api_base { 
    public:
        virtual int bar() = 0;
    };

    class imp_base { 
    public:
        int foo() { printf("foo from imp_base\n"); } 
    };

    class imp_top : public api_top, public imp_base { 
    public:
        int bar() { printf("bar from imp_top\n"); } 
    };

    int main()
    { 
       printf("Hello\n");                                                                                                                                                                                                                                                   
       imp_top a;

       a.bar();
       a.foo();

       return 1;
    }

Compiler result:

    test.cpp:26:12: error: cannot declare variable ‘a’ to be of abstract type ‘imp_top’                                                                                                                                                                                     
        imp_top a;
                ^
    test.cpp:18:7: note:   because the following virtual functions are pure within ‘imp_top’:
     class imp_top : public api_top, public imp_base {
           ^
    test.cpp:5:17: note:  virtual int api_base::foo()
         virtual int foo() = 0;
                     ^
    test.cpp:29:6: error: request for member ‘foo’ is ambiguous
        a.foo();
          ^
    test.cpp:15:9: note: candidates are: int imp_base::foo()
         int foo() { printf("foo from imp_base\n"); }
             ^
    test.cpp:5:17: note:                 virtual int api_base::foo()
         virtual int foo() = 0;

Answer 1

Change:

class api_top : public api_base to class api_top : public virtual api_base

and:

class imp_base to class imp_base : public virtual api_base

Then it works.

To understand this, see: virtual inheritance. And yes (just saw Ext3h's post), use the override keyword.