如何在保持我的实现者类私有的同时在C中使用可变参数模板？

#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\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 v4 ) : builtin_pack ( v1,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;
}

该代码编译错误：

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>' 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>' requested here
    p3.apply ( [] ( long i,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.

我想要做的是使用自定义(lambda)函数(称为“apply”)实现映射方法.当私有实现者 – 公共包装器的层次结构不存在时,它很容易工作,所以当数组v只在类包中时,它会按预期编译和运行.但是,当数据存储在私有继承的类中时,它不起作用.

该类的结构是一个私有的实现者类以及一个包装类,在中间我遇到了这个错误.

我是否以错误的方式使用了可变参数模板？或者有可用的解决方法吗？

(抱歉我的表情很差,因为我是C的新手和stackoverflow以及非母语的英语,只要保留原意,欢迎修改或提出问题！)