矩阵类的 C++ 自定义迭代器

Question

我有一个 Matrix 类，现在我想为它编写一个迭代器。我的类(class)看起来像这样:

template <typename T>
class Matrix {
public:
vector<vector<T>> data;
Matrix(const vector<vector<T>>& d)
        : data(d) {
}
std::pair<size_t, size_t> size() const {
    std::pair<size_t, size_t> s;
    s.first = data.size();
    s.second = data[0].size();
    return s;
}

我想为它写一个迭代器。但是我该怎么做呢？我找到了以下代码:

class iterator
    {
        public:
            typedef iterator self_type;
            typedef T value_type;
            typedef T& reference;
            typedef T* pointer;
            typedef std::forward_iterator_tag iterator_category;
            typedef int difference_type;
            iterator(pointer ptr) : ptr_(ptr) { }
            self_type operator++() { self_type i = *this; ptr_++; return i; }
            self_type operator++(int junk) { ptr_++; return *this; }
            reference operator*() { return *ptr_; }
            pointer operator->() { return ptr_; }
            bool operator==(const self_type& rhs) { return ptr_ == rhs.ptr_; }
            bool operator!=(const self_type& rhs) { return ptr_ != rhs.ptr_; }
        private:
            pointer ptr_;
    };

 iterator begin()
    {
        return iterator(data_);
    }

    iterator end()
    {
        return iterator(data_ + size_);
    }

我怎样才能让它协同工作？我想遍历矩阵中的每个元素

Answer 1

这里是没有提升的 C++ 解决方案。我还提供了完整且经过测试的源代码示例。

源码使用MS Visual Studio 19编译测试。

第一个提示:我总是使用 std::valarray 进行矩阵计算。请阅读它。

对该解决方案的解释:

我们将使用 int vector 的 vector 来表示矩阵。可以轻松访问行。它们是数据矩阵的第一维。如果我们想要一行的迭代器，我们简单地返回一个 vector 的标准迭代器。这样，我们将立即拥有完整的功能。简单。

不幸的是列不同。它们是数据连续内存中的切片。因此，我们将实现的解决方案是:为每一列创建一个 vector ，并引用正确位置的数据。

这听起来比实际更容易，因为我们不能在 C++ 中的容器中存储引用。因此，要么使用 std::reference_wrapper，要么构建我们自己的引用包装器。我在为取消引用的 std::reference_wrapper 赋值并构建自己的值时遇到了问题。添加了赋值运算符。

有了它，我们可以根据引用 vector 将迭代器返回到列。

可以添加许多附加功能。当然，您可以将 int 替换为任何其他值或将其模板化。

我在main里放了一些测试代码

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <tuple>
#include <sstream>
#include <numeric>


// Unfortunately the std::reference_wrapper does not work as expected.
// So we will build our own one
class IntRef
{
    // Here we will store the reference
    std::tuple<int&> t;
public:
    // Constructor. Take reference and store it in tuple
    IntRef(int&& intV) : t(intV) {}     

    // Assignment to the referenced value
    int operator =(const int i) { std::get<0>(t) = i; return i; }      

    // Explicit type cast to int&
    operator int& () { return std::get<0>(t); }                        

    // And, return the reference
    decltype(&std::get<0>(t)) operator&() { return &std::get<0>(t); }  
};


// Some definitions to make reading easier
using IntRefV = std::vector<IntRef>;
using MatrixCIterator = std::vector<IntRef>::iterator;
using Columns = std::vector<int>;
using MatrixRIterator = Columns::iterator;


// The matrix
class Matrix
{
public:
    // Constructor defines the matrix size
    Matrix(size_t numberOfRows, size_t numberOfColumns);

    // Iterators for rows are simple, becuase we have vectors of columns. Use unterlying iterator
    MatrixRIterator rowIterBegin(size_t row) { return data[row].begin(); }
    MatrixRIterator rowIterEnd(size_t row) { return data[row].end(); }  

    // Column iterator is complicated. Retzurn iterator to vevtor of references to column values
    MatrixCIterator columnIterBegin(size_t column) { return columnReferences[column].begin(); }
    MatrixCIterator columnIterEnd(size_t column) { return columnReferences[column].end(); }

    // Access data of matrix
    std::vector<int>& operator [] (const size_t row) { return data[row]; }

    // And, for debug purposes. Output all data
    friend std::ostream& operator << (std::ostream& os, const Matrix& m) {
        std::for_each(m.data.begin(), m.data.end(), [&os](const Columns& columns) {std::copy(columns.begin(), columns.end(), std::ostream_iterator<int>(os, " ")); std::cout << '\n'; });
        return os;
    }
protected:
    //The matrix, vector of vector of int
    std::vector<Columns> data; 

    // The references to columns in data
    std::vector<IntRefV> columnReferences{};    
};

// Constructor. Build basic matrix and then store references to columns in data 
Matrix::Matrix(size_t numberOfRows, size_t numberOfColumns) : data(numberOfRows, std::vector<int>(numberOfColumns)), columnReferences(numberOfColumns)
{
    for (size_t column = 0; column < numberOfColumns; ++column) 
        for (size_t row = 0; row < numberOfRows; ++row)
            columnReferences[column].emplace_back(IntRef(std::move(data[row][column]))); // Std::move creates a rvalue reference (needed for constructor, nothing will be moved)
}



// Some test data for the istream_iterator
std::istringstream testData("1 2 10");



// Test the matrix
int main()
{
    // Define a matrix with 3 rows and 4 columns
    Matrix matrix(3, 4);
    // Test 1: Fill all values in column 2 with 42
    for (MatrixCIterator ci = matrix.columnIterBegin(2); ci != matrix.columnIterEnd(2); ++ci) {
        *ci = 42;
    }
    std::cout << matrix << "Column 2 filled with 42\n\n";

    // Test 2: Read input from istream and copy put that in column 1
    std::copy_n(std::istream_iterator<int>(testData), 3, matrix.columnIterBegin(1));
    std::cout << matrix << "Column 1 filled with testData '"<< testData.str() << "'\n\n";

    // Test 3: Copy column 2 to cout (Print column 2)
    std::copy(matrix.columnIterBegin(2), matrix.columnIterEnd(2), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "This is column 2\n\n";

    // Test 4: Sum up the first 2 values of column 1 and show result
    std::cout << "\nSum of first 2 values of column 1:  " << std::accumulate(matrix.columnIterBegin(1), matrix.columnIterBegin(1)+2, 0) << "\n\n";

    // Test 5: Fill all values in row 0 with 33
    std::for_each(matrix.rowIterBegin(0), matrix.rowIterEnd(0), [](int& i) { i = 33; });
    std::cout << matrix << "Row 0 filled with 33\n\n";

    return 0;
}

希望这能让您了解它的工作原理。 . .