c++ - 具有 C++ 迭代器参数的多功能函数

Question

我正在使用通过迭代器接受范围的函数，类似于以下代码中的“printPoints”和“printPoints2”。到目前为止，“printPoints”可以接受 Point 对象的 vector/列表/等的迭代器，但需要“printPoints2”来处理指向 Point 对象的指针的 vector/列表/等。有什么技巧可以编写一个更通用的函数来替换这两个函数吗？

提前致谢。

#include <iostream>
#include <vector>
#include <list>
#include <iterator>
#include <memory>

struct Point {
    int x;
    int y;

    Point(int x, int y): x(x), y(y) {}
};

/*
 Is there a more versatile function to replace the following two?
 */
template <class Iter>
void printPoints(Iter begin, Iter end) {
    for(auto it=begin; it!=end; ++it)
        std::cout << "{" << it->x << " " << it->y << "}";
}
template <class Iter>
void printPoints2(Iter begin, Iter end) {
    for(auto it=begin; it!=end; ++it)
        std::cout << "{" << (*it)->x << " " << (*it)->y << "}";
}

int main()
{
    std::vector<Point> vecPoints = {{0,0}, {1,1}};
    std::cout << "vector of points: ";
    printPoints(vecPoints.begin(), vecPoints.end());
    std::cout << "\n";

    std::list<Point> listPoints = {{2,2}, {3,3}};
    std::cout << "list of points: ";
    printPoints(listPoints.begin(), listPoints.end());
    std::cout << "\n";

    std::vector<std::unique_ptr<Point>> vecPtrPoints;
    vecPtrPoints.push_back(std::make_unique<Point>(4,4));
    vecPtrPoints.push_back(std::make_unique<Point>(5,5));
    std::cout << "vector of pointers to point: ";

    // won't work because of "it->x" inside the function
    //printPoints(vecPtrPoints.begin(), vecPtrPoints.end());
    printPoints2(vecPtrPoints.begin(), vecPtrPoints.end());
    std::cout << "\n";
}

Answer 1

C++17 助你一臂之力!

#include <type_traits>

template <class Iter>
void printPoints(Iter begin, Iter end) {
    for(auto it=begin; it!=end; ++it)
    {
        if constexpr (std::is_same_v<typename std::iterator_traits<Iter>::value_type, Point>)
        {
            std::cout << "{" << it->x << " " << it->y << "}";
        }
        else
        {
            std::cout << "{" << (*it)->x << " " << (*it)->y << "}";
        }
    }
}

如果你没有 c++17 那么你可以通过使用 std::enable_if 来实现类似的东西，让你有两个 printPoints 函数来同名。

另一种方法是重构您的代码:

void printPoint(const Point& point)
{
    std::cout << "{" << point.x << " " << point.y << "}";
}

void printPoint(const std::unique_ptr<Point>& point)
{
    printPoint(*point);
}

template <class Iter>
void printPoints(Iter begin, Iter end) {
    for(auto it=begin; it!=end; ++it)
    {
        printPoint(*it);
    }
}

这有点冗长，但适用于早期的 c++ 标准，对于新手 c++ 程序员来说可能更容易理解。

选项 3 是两者的结合:

void printPoint(const Point& point)
{
    std::cout << "{" << point.x << " " << point.y << "}";
}

template <class Iter>
void printPoints(Iter begin, Iter end) {
    for(auto it=begin; it!=end; ++it)
    {
        if constexpr (std::is_same_v<typename std::iterator_traits<Iter>::value_type, Point>)
        {
            printPoint(*it);
        }
        else
        {
            printPoint(**it);
        }
    }
}