我如何在c++中使用可变模板，同时保持我的实现类私有

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

本文关键字：我的 实现 c++      更新时间：2023-10-16
#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,%lxn", 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;
}

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.

我想做的是实现一个映射方法与自定义(lambda)函数(称为'apply')。当私有实实者-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，当数据存储在私有继承的类中时，它不起作用。

类的结构是一个私有实现者类和一个包装器类，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模板?或者是否有可用的变通方法?

(很抱歉我的表达不佳，因为我是c++和stackoverflow的新手，母语不是英语，只要保留原意，欢迎对问题进行修改或建议!)

您的问题是使用私有继承，您无法从pack*转换为builtin_pack*(即pack之外)。如果你转换它，代码编译，虽然我不确定这是否是你所追求的:

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

现场演示

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