How can I use variadic template in C++ while keeping my implementor class private?

Question

#include <cstdio>

class builtin_pack
{
    long v[4];
public:
    builtin_pack ( long v1, long v2, long v3, long v4 ) : v{v1, v2, v3, v4} {}
    void builtin_op()
    {
        printf ( "%lx,%lx,%lx,%lx\n", v[0], v[1], v[2], v[3] );
    };
    template<typename Func, typename... Targs>
    void builtin_apply ( Func f, Targs ... t )
    {
        for ( int i = 0; i < 4; i++ )
        {
            v[i] = f ( t.v[i]... );
        }
    }
};

class pack : builtin_pack
{
public:
    pack ( long v1, long v2, long v3, long v4 ) : builtin_pack ( v1, v2, v3, v4 ) {}
    template<typename Func, typename... Targs>
    pack& apply ( Func f, Targs ... t )
    {
        this->builtin_apply ( f, t... );
        return *this;
    }
    void op()
    {
        this->builtin_op();
    }
};

int main()
{
    pack p1{0xff, 0x0f, 0xf0, 0x06}, p2{0x0f00, 0xf000, 0x6700, 0xff00};
    pack p3{0x12340000, 0x56780000, 0x45120000, 0xdead0000};
    p3.apply ( [] ( long i, long j, long k )->long{return i | j | k;}, p1, p2, p3 );
    p3.op();
    return 0;
}

That code compiles with an error:

main.cpp:17:24: error: cannot cast 'pack' to its private base class 'builtin_pack'
            v[i] = f ( t.v[i]... );
                       ^
main.cpp:29:15: note: in instantiation of function template specialization 'builtin_pack::builtin_apply<(lambda
      at main.cpp:42:16), pack, pack, pack>' requested here
        this->builtin_apply ( f, t... );
              ^
main.cpp:42:8: note: in instantiation of function template specialization 'pack::apply<(lambda at
      main.cpp:42:16), pack, pack, pack>' requested here
    p3.apply ( [] ( long i, long j, long k )->long{return i | j | k;}, p1, p2, p3 );
       ^
main.cpp:22:14: note: implicitly declared private here
class pack : builtin_pack
             ^~~~~~~~~~~~
main.cpp:17:26: error: 'v' is a private member of 'builtin_pack'
            v[i] = f ( t.v[i]... );
                         ^
main.cpp:22:14: note: constrained by implicitly private inheritance here
class pack : builtin_pack
             ^~~~~~~~~~~~
main.cpp:5:10: note: member is declared here
    long v[4];
         ^
2 errors generated.

What I want to do is to implement a mapping method with a custom (lambda) function (called 'apply'). It works readily when the hierarchy of private implementor-public wrapper is absent, so when the array v is just in the class pack, it compiles and runs as expected. However, it does not work when data is stored in a private-inherited class.

The structure of the class is a private implementor class along with a wrapper class, and in the middle I encountered this error.

Have I used the variadic template the wrong way? Or is there a workaround available?

(Sorry for my poor expression as I am a newbie in C++ and stackoverflow and non-native speaker of English, and modification or suggestion of the question is welcomed as long as the original intention is reserved!)

Answer 1

Your problem is that with private inheritance, you cannot convert from pack* to builtin_pack* (outside of pack, that is). If you do cast it, the code compiles, although I'm not sure if this is what you were after:

template<typename Func, typename... Targs>
pack& apply ( Func f, Targs ... t )
{
    this->builtin_apply ( f, static_cast<builtin_pack&&>(t)... );
    return *this;
}

live demo