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

c++, c++11, templates, variadic-templates

Solution

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

Problem

``` #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!)

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