如何转发 C++基地和通过模板构造函数的成员初始值设定项？

Question

我想制作一个模板，其参数 B 和 D 是模板实例化中固定数组的基类和元素：

template <class B,class D>
struct fix_array : public B {
    D    myArray[5];
    /**/ fix_array( void ) { cout << "completely default constructed"; }
};

这工作正常。接下来，我想将初始化值传递给 B：

template <class... BI>
/**/    fix_array( BI &&... bi )
           : B( std::forward<BI>(bi)... )
           , myArray( )
           {  cout << "This works too, even for multi-argument constructors of B."; }

这也有效。最后，我的问题：

如何通过模板构造函数将初始化值传递给 D myArray[5];？

我设想调用代码看起来像这样：

fix_array<B,D>  myVal( { B-initializers }, { { D[0]-initializers }, { D[1]-initializers } } );

但我迷失在可变参数模板地狱中。所有 D 初始化器都必须具有相同的形式吗？ D 可以用 int、const char * 或 D && 初始化。）

（假设 是我的测试程序。 debug-util.hh 定义了一个类 Vobject，它在构造、析构、复制等时打印出信息。

#include <stdio.h>
#include <memory.h>
#include <iostream>
#include <typeinfo>
#include <memory>
#include <vector>

#include "debug-util.hh"     // Defines Vobject and some other info-printing utilities

template <class B, class T>
class fix_array : public B
{
  public:
    T            pData[5];

    /**/         fix_array( void ) : B( ), pData( ) { std::cerr << "completely default construction" << std::endl; }

    template <class... BI>
    /**/         fix_array( BI &&... bi )
                   : B( std::forward<BI>(bi)... )
                   , pData( )
                   {
                       std::cerr << "fix_array Base was initialized" << std:: endl;
                   }
} ;

/*
 * A Vobject has 4 constructors: (void), (int), (int,char*), (int,int,int)
 *
 * It prints information about how it's constructed, when it's copied or moved,
 * and when it's destroyed.
 */

int
main( int, char ** )
{
    fix_array<Vobject,Vobject>   a;                 // These 4 initialize the Base Vobject
    fix_array<Vobject,Vobject>   b( 1 );            // using the various constructors.
    fix_array<Vobject,Vobject>   c( 3, "foobar" );
    fix_array<Vobject,Vobject>   d( 100, 200, 300 );

    // HOW CAN I PASS INITIALIZERS TO THE pDATA ARRAY MEMBERS???

    // fix_array<Vobject,Vobject> x( { 123, "base init" }, { { 1 }, { 2, "xyz" }, { 2, 4, 5 } } );

    std::cerr << "Hello, world" << std::endl;

    return 0;
}

（是的，我知道这是一个愚蠢的程序，它使用 std::vector 等会更聪明，但我有一个更复杂的最终目标，学习如何使用可变参数模板将帮助我实现它。）

该程序的执行：

Vobject@A000()
Vobject@A001()
Vobject@A002()
Vobject@A003()
Vobject@A004()
Vobject@A005()
completely default construction
Vobject@A006(1)
Vobject@A007()
Vobject@A008()
Vobject@A009()
Vobject@A010()
Vobject@A011()
fix_array Base was initialized
Vobject@A012(3, "foobar")
Vobject@A013()
Vobject@A014()
Vobject@A015()
Vobject@A016()
Vobject@A017()
fix_array Base was initialized
Vobject@A018(100,200,300)
Vobject@A019()
Vobject@A020()
Vobject@A021()
Vobject@A022()
Vobject@A023()
fix_array Base was initialized
Hello, world
~Vobject@A023 (was constructed Vobject(void))
~Vobject@A022 (was constructed Vobject(void))
~Vobject@A021 (was constructed Vobject(void))
~Vobject@A020 (was constructed Vobject(void))
~Vobject@A019 (was constructed Vobject(void))
~Vobject@A018 (was constructed Vobject(int,int,int))
~Vobject@A017 (was constructed Vobject(void))
~Vobject@A016 (was constructed Vobject(void))
~Vobject@A015 (was constructed Vobject(void))
~Vobject@A014 (was constructed Vobject(void))
~Vobject@A013 (was constructed Vobject(void))
~Vobject@A012 (was constructed Vobject(int,const char*))
~Vobject@A011 (was constructed Vobject(void))
~Vobject@A010 (was constructed Vobject(void))
~Vobject@A009 (was constructed Vobject(void))
~Vobject@A008 (was constructed Vobject(void))
~Vobject@A007 (was constructed Vobject(void))
~Vobject@A006 (was constructed Vobject(int))
~Vobject@A005 (was constructed Vobject(void))
~Vobject@A004 (was constructed Vobject(void))
~Vobject@A003 (was constructed Vobject(void))
~Vobject@A002 (was constructed Vobject(void))
~Vobject@A001 (was constructed Vobject(void))
~Vobject@A000 (was constructed Vobject(void))

Answer 1

我怀疑你在试图同时传递两包时被卡住了。您可能会被困在那里，因为这是直观的事情。不幸的是，直觉在这里让我们失败了，因为从裸函数参数中进行包推导的行为方式是直觉所预料不到的。尾随包的行为不同，很难知道第二个包何时开始以及第一个包何时结束，并且一路上出现的错误并不总是很清楚。

在这种情况下，我喜欢简单地执行标准库所做的事情。 std::pair 是聚合两个元素的类型。它允许通过将一组参数传递给每个元素来单独（就地）初始化每个元素（构造函数 8)。这与 std::forward_as_tuple 协同作用，一个标准的功能模板，用于转发。您可以通过执行类似的操作向基数和数组提供参数。

剩下的问题只是如何提取元组元素。 B 在 C++17 的 std 中很简单： :make_from_tuple。 myArray 没有同样的好处，因为 c 样式数组是不规则类型。然而，我们可以使用委托构造函数和实用程序 std::index_sequence 来构造数组。我也将在 B 中使用这种方法，因此总共只需要 C++14。

template <class B, class T>
class fix_array : public B
{
public:
    template <class... BRefs, class... DRefs>
    fix_array(std::tuple<BRefs...> bArgs, std::tuple<DRefs...> dArgs)
    : fix_array(std::move(bArgs), std::make_index_sequence<sizeof...(BRefs)>{}, std::move(dArgs), std::make_index_sequence<sizeof...(DRefs)>{})
    {}

private:
    T            pData[5];

    template <class... BRefs, std::size_t... Bidx, class... DRefs, std::size_t... Didx>
    fix_array(std::tuple<BRefs...> bArgs, std::index_sequence<Bidx...>, std::tuple<DRefs...> dArgs,  std::index_sequence<Didx...>)
    : B( std::get<Bidx>(bArgs)... )
    , pData{ std::get<Didx>(dArgs)... }
    {
        std::cerr << "fix_array Base was initialized" << std:: endl;
    }
};

两个 index_sequence 参数是标记类型占位符，其中按顺序包含每个元组元素的索引。它们只是在那里扩展并访问正确序数的每个元组元素。在 std::forward_as_tuple 完成其工作后，std::get 已经通过正确的引用类型提取元素。

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