c++ - 如何在 C++ 中使用可变参数模板同时保持我的实现类私有(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;
}

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, 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.

我想做的是使用自定义 (lambda) 函数(称为“应用”)实现映射方法。当私有(private)实现者-公共(public)包装器的层次结构不存在时，它很容易工作，因此当数组 v 刚好在类 pack 中时，它会按预期编译和运行。但是，当数据存储在私有(private)继承类中时，它不起作用。

该类的结构是一个私有(private)实现类和一个包装类，中间遇到了这个错误。

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？

(由于本人是C++和stackoverflow新手，英语非母语，所以表达不好请见谅，在不忘初心的前提下，欢迎对问题提出修改或建议!)

Answer 1

您的问题是，对于私有(private)继承，您无法从 pack* 转换为 builtin_pack*(即在 pack 之外)。如果你确实转换它，代码会编译，虽然我不确定这是否是你想要的:

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

live demo