c++ - 如何遍历 std::index_sequence

Question

我的源代码中有这段代码:

template <std::size_t... Dims>
class DimensionPack {
public: 
    using Dimensions = std::index_sequence<Dims...>;
    static const std::size_t total_dimensions = sizeof...(Dims);
    std::vector<unsigned int> even_or_odd;

public:
    DimensionPack() {
        unsigned idx = 0;
        for ( ; idx < total_dimensions; idx++ ) {
            //MatrixDimensionOddOrEven mdoe( Dimensions[idx] );
            //unsigned int val = mdoe.even_or_odd;
            //even_or_odd.push_back( val );
        }
    }
};

注释的代码行是有问题的代码。我不熟悉使用 std::indx_sequence<>我已经阅读了 MSD 的文档，但仍然不确定，尤其是当它与 using 一起使用时指令。

此模板类将用作其他可变参数模板类的参数包，用于从可变参数列表中存储和提取数据。在此类的构造函数中，我使用此结构的构造函数和私有(private)方法来检查值并返回其偶数或奇数状态，并从其公共(public)常量成员获取值:

struct MatrixDimensionOddOrEven {
    const unsigned int even_or_odd; 
    explicit MatrixDimensionOddOrEven( unsigned int odd_or_even ) : even_or_odd( test( odd_or_even ) ) {}

private:
    const unsigned int test( unsigned int value ) const {
        if ( value == 0 ) {
            std::ostringstream strStream;
            strStream << __FUNCTION__ << "invalid number: " << value << " must be >= 1.";
            Logger::log( strStream, Logger::TYPE_ERROR );
            throw ExceptionHandler( strStream );
        }
        return ( ((value % 2) == 0) ? ODD : EVEN );
    }
};

这个头文件的cpp文件可以编译，但是当我去编译另一个包含它的cpp文件比如main时；编译失败。

这是我收到的当前错误消息:

1>------ Build started: Project: FileTester, Configuration: Debug Win32 ------
1>  main.cpp
1>c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(44): error C2540: non-constant expression as array bound
1>  c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(41): note: while compiling class template member function 'DimensionPack<2,3,4>::DimensionPack(void)'
1>  c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\main.cpp(336): note: see reference to function template instantiation 'DimensionPack<2,3,4>::DimensionPack(void)' being compiled
1>  c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(60): note: see reference to class template instantiation 'DimensionPack<2,3,4>' being compiled
1>  c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\main.cpp(155): note: see reference to class template instantiation 'Matrix<float,2,3,4>' being compiled
1>c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(45): error C2228: left of '.even_or_odd' must have class/struct/union
1>c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(45): error C2789: 'val': an object of const-qualified type must be initialized
1>  c:\users\skilz80\documents\visual studio 2015\projects\filetester\filetester\matrix.h(45): note: see declaration of 'val'
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

给我带来麻烦的不是编译器错误，而是编译器错误。在可变参数模板和参数包的使用中，更多关于这种语法对我来说有点新。那么我该如何正确编写语法来获取这个 std::index_sequence<...> 的各个元素呢？分配给 Dimensions与 using指令以便将这些值传递给在 DimensionPack 类的 for 循环中看到的辅助结构的构造函数？

Answer 1

不需要index_sequence。这将做到:

template <std::size_t... Dims>
class DimensionPack {
public:
    std::vector<unsigned int> even_or_odd;

public:
    DimensionPack()
        : even_or_odd{MatrixDimensionOddOrEven{Dims}.even_or_odd...}
    {
    }
};

作为奖励，它的美妙之处在于您不需要push_back 每个元素。您可以直接使用所有元素初始化 vector 。