range-for循环到底能做什么？

Question

I'm working on a snake game program. 我正在开发蛇游戏程序。 I use a deque of Body in class Snake to represent a snake and of course Body is a struct I have defined. 我在Snake类中使用Body的双端队列来表示蛇，当然Body是我定义的结构。 Here is part of the code: 这是代码的一部分：

struct Body {        // one part of snake body
    int x, y, direction;
    Body() : x(0), y(0), direction(UP) { }
    Body(int ix, int iy, int id) : x(ix), y(iy), direction(id) { }
};

class Snake {
protected:
    std::deque<Body> body;
    // other members
public:
    auto begin()->std::deque<Body>::const_iterator const { return body.cbegin(); }
    auto end()->std::deque<Body>::const_iterator const { return body.cend(); }
    // other members
};

And in another function construct_random_food I need to generate a food and make sure it does not coincide with the snake. 在另一个函数construct_random_food中，我需要生成一个食物，并确保它与蛇不重合。 Here's the function definition: 这是函数定义：

Food construct_random_food(int gameSize, const Snake& snake) {
    static std::random_device rd;
    static std::uniform_int_distribution<> u(2, gameSize + 1);
    static std::default_random_engine e(rd());
    Food f;
    while (1) {
        f.x = u(e) * 2 - 1;
        f.y = u(e);
        bool coincide = 0;
        for (const auto& bd : snake) // This causes an error.
            if (bd.x == f.x && bd.y == f.y) {
                coincide = 1; break;
            }
        if (!coincide) break;
    }
    return f;
}

An error is caused at the range-based for-loops line. 在基于范围的for循环行中导致错误。 It says that I'm trying to cast const Snake to Snake& (casting a low-level const away). 它表示我正在尝试将const Snake强制转换为Snake＆（将低级const转换掉）。 I fix the problem by rewriting that line like this: 我通过重写该行来解决此问题：

for (const auto& fd : const_cast<Snake&>(snake))

So I'm wondering what exactly a range-for do and what it needs. 所以我想知道范围是做什么的，它需要什么。 Does the error have anything to do with the begin() function in class Snake? 该错误与Snake类中的begin（）函数有关吗？

Answer 1

The problem is that your begin and end functions are not const. 问题是您的begin和end函数不是const。

auto begin()->std::deque<Body>::const_iterator const { return body.cbegin(); }
            // this applies to the return type ^^^^^

You've applied the const qualifier to the return type, not to the calling object. 您已将const限定符应用于返回类型，而不是调用对象。 Put the const qualifier before the trailing return type. 将const限定词放在尾随返回类型之前。

auto begin() const ->std::deque<Body>::const_iterator { return body.cbegin(); }

You can see the correct order in which you should place function qualifiers here: http://en.cppreference.com/w/cpp/language/function 您可以在此处看到放置函数限定符的正确顺序： http : //en.cppreference.com/w/cpp/language/function

range-for循环到底能做什么？

问题描述

1 个解决方案

解决方案1
6 已采纳 2017-08-11 16:47:36

range-for循环到底能做什么？

问题描述

1 个解决方案

解决方案1 6 已采纳 2017-08-11 16:47:36

解决方案1
6 已采纳 2017-08-11 16:47:36