class I1
{
public:
    virtual void method1() = 0;
};

class I2
{
public:
    virtual void method2() = 0;
};

class A : public I1, public I2
{
public:
    int id;
    virtual void method1(){}
    virtual void method2(){}
};

class C
{
public:
    std::vector<A> vector;

    I1* getI1ById(int id){return &(vector[id]);}
    I2* getI2ById(int id){return &(vector[id]);}
};

int main()
{
    C c;
    A a;

    c.vector.push_back(a);
    //...

    I1* i1 = c.getI1ById(0);
    I2* i2 = c.getI2ById(0);

    a.method1(); // ok
    a.method2(); // ok

    i1->method1(); // ok
    i1->method2(); // error

    i2->method1(); //error
    i2->method2(); //ok

  return 0;
}