c++ - 如何遍历结构中的 vector ？

我正在为学校创建各种数字自动售货机，但遇到了问题。我创建了一个Items结构来进行自动售货。然后，我创建了一个名为Machine的结构，其中包含一个vector<Items>。我想创建一个for循环，该循环遍历vector<Item>并显示项目，但出现以下错误:

C:\Users\Nate\Desktop>g++ structversion.cpp -o structversion.exe -std=c++11
structversion.cpp: In function 'int test(Machine)':
structversion.cpp:29:20: error: 'begin' was not declared in this scope
   for (Item item : machine) {
                    ^
structversion.cpp:29:20: note: suggested alternatives:
In file included from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/string:51:0,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/locale_classes.h:40,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/ios_base.h:41,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/ios:42,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/ostream:38,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/iostream:39,
                 from structversion.cpp:1:
C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/range_access.h:87:5: note:   'std::begin'
     begin(_Tp (&__arr)[_Nm])
     ^
C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/range_access.h:87:5: note:   'std::begin'
structversion.cpp:29:20: error: 'end' was not declared in this scope
   for (Item item : machine) {
                    ^
structversion.cpp:29:20: note: suggested alternatives:
In file included from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/string:51:0,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/locale_classes.h:40,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/ios_base.h:41,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/ios:42,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/ostream:38,
                 from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/iostream:39,
                 from structversion.cpp:1:
C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/range_access.h:97:5: note:   'std::end'
     end(_Tp (&__arr)[_Nm])
     ^
C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/range_access.h:97:5: note:   'std::end'

抱歉，这是一个多余或愚蠢的问题。这也是有争议的代码:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

struct Item
{
    string name;
    double price;
    unsigned int quantity;
    unsigned int amountInCart;
    bool addedToCart;
};

struct Machine { vector<Item> menu; };

void initItem(Item& i, string name, double price, unsigned int quantity,
    unsigned int amountInCart, bool addedToCart)
{
    i.name = name;
    i.price = price;
    i.quantity = quantity;
    i.amountInCart = amountInCart;
    i.addedToCart = addedToCart;
}

test(Machine machine)
{
    for (Item i : machine) {
        cout << "item = " << i.name;
    }
}

main()
{
    Item cake;
    Item fruit;
    Item chips;
    Item soda;
    Item juice;

    initItem(cake, "Cake", 3.00, 5, 0, false);
    initItem(fruit, "Fruit", 4.20, 15, 0, false);
    initItem(chips, "Chips", 1.00, 6, 0, false);
    initItem(soda, "Soda", 1.50, 7, 0, false);
    initItem(juice, "Juice", 1.90, 10, 0, false);

    Machine machine;
    machine.menu.push_back(cake);
    machine.menu.push_back(fruit);
    machine.menu.push_back(chips);
    machine.menu.push_back(soda);
    machine.menu.push_back(juice);

    test(machine);
    return 0;
}

我尝试在函数test上迭代Machine结构内的 vector 菜单。

我很新，所以如果有人有时间并且可以ELI5我做错了，那将是惊人的。

最佳答案

为了在用户定义的类型中使用range-based for loop，您需要定义begin()和end()迭代器。这意味着Machine需要具有以下成员函数，这些成员函数返回成员容器的迭代器。 (See live online)

struct Machine
{
    vector<Item> menu;
    auto begin() { return menu.begin(); } // auto return requires C++14
    auto end()   { return menu.end();   }

    // or in C++11, you provide the correct return type
    // decltype(menu.begin()) begin() { return menu.begin(); }
    // decltype(menu.end())   end()   { return menu.end();   }
};

否则，您需要直接向循环提供迭代对象。在您的情况下，这意味着为了进行最小的更改:

void test(Machine const& machine) // pass by `const-ref` as the machine is read-only inside the function
{
    for (const Item& i : machine.menu) { // const& : same reason as above ^^
    //                   ^^^^^^^^^^^^ --> std::vector has iterators: begin(), end()
        cout << "item = " << i.name;
    }
}

其他注意事项:

initItem函数不是您在C++(或任何其他面向对象的编程语言)中通常所采用的方法。那是constructor的工作。为了更改对象中成员的值，将使用setter(成员)功能。一个好的开始将是:

class Item
{
private:
    std::string name;
    double price;
    unsigned int quantity;
    unsigned int amountInCart;
    bool addedToCart;

public:
    // constructor
    Item(std::string name, double price, unsigned int quantity,
        unsigned int amountInCart, bool addedToCart)
        : name{ name } // or name{ std::move(name) } when you learn about the move-semantics
        , price{ price}
        , quantity{ quantity }
        , amountInCart{ amountInCart }
        , addedToCart{ addedToCart }
    {}
    // ... getter and setter functions
};

您现在创建一个对象，如下所示:

Item cake{ "Cake", 3.00, 5, 0, false };

请不要练习using namespace std;。有关更多信息，请参见此处:Why is "using namespace std;" considered bad practice?

我已将Machine对象传递给test合格引用传递的函数Item(也在基于范围的循环中，const对象)。无论何时，数据在任何范围内都是不可修改的，您应该传递这样的参数，以便避免不必要的复制(creadits @Klaus):。进一步的阅读:Is it better in C++ to pass by value or pass by constant reference?