c++ - 无法访问 std::map 中的结构值

Question

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Answer 1

这在技术上可以被认为是 Read the Documentation 的一个案例，我这样做是为了自己解决这个问题。但是，解决方案可能并非一蹴而就。

Forecast 结构体有一个构造函数，但它是一个需要两个参数的构造函数。这会导致与 map 的 [] 运算符的一个主要行为发生冲突，而迭代器访问则没有:

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value. Notice that this always increases the container size by one, even if no mapped value is assigned to the element (the element is constructed using its default constructor).

因为 Forecast 没有这样的默认构造函数，map 试图调用一个不存在的函数。

如果我在 Forecast 上省略了构造函数，编译器就会为该结构提供一个默认构造函数，一切都会好起来的。但是，由于我定义了一个构造函数，编译器不会那样做。

值得庆幸的是，解决方案非常简单:要么删除构造函数，要么添加默认构造函数。就我而言，我可以设置默认参数。

struct Temps
{
    Temps(int l = 0, int h = 0)
        :low(l), high(h)
        {}
    int low;
    int high;
};

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NOTE: Some may argue that the above example is entirely silly - why not just use the iterator? In this case, that would be a very good idea. Using the [] is probably just wasting time, since I'm searching for one anyway to prevent errors. However, there are cases where [] is the best operator for the job, even if this isn't one.