如果我读取地图的值，其中键不存在会发生什么？ [英] What happens if I read a map's value where the key does not exist?

问题描述

  map< string，string>达达
 dada [dummy] =papy; 
 cout<< dao [pootoo]; 
  

我感到困惑，因为我不知道它是否被认为是未定义的行为，

解决方案

地图:: operator [] 在数据结构中搜索与给定键对应的值，并返回对该键的引用。

找到一个它透明地为它创建一个默认构造的元素。 （如果你不想要这个行为，你可以使用 map :: at 函数。）

在此处获取std :: map的完整方法列表：

http://en.cppreference.com/w/cpp/container/map

这是地图的文档:: operator [] 从当前C ++标准...

23.4.4.3地图元素访问

T&运算符[]（const key_type& x）;

2. 需要：key_type必须为CopyConstructible，而mapped_type必须为DefaultConstructible。 / p>




3. 返回：与*中对应的mapped_type的引用。

：对数。

operator []（key_type&& x）;

1. 效果：相当于地图中的x，将VALUE_type（std :: move（x），T（））插入到地图中。




2. 需要：mapped_type应为DefaultConstructible 。




3. 返回：对应于x中的x的mapped_type的引用。




4. 复杂性：对数。

map<string, string> dada;
dada["dummy"] = "papy";
cout << dada["pootoo"];

I'm puzzled because I don't know if it's considered undefined behaviour or not, how to know when I request a key which does not exist, do I just use find instead ?

解决方案

The map::operator[] searches the data structure for a value corresponding to the given key, and returns a reference to it.

If it can't find one it transparently creates a default constructed element for it. (If you do not want this behaviour you can use the map::at function instead.)

You can get a full list of methods of std::map here:

http://en.cppreference.com/w/cpp/container/map

Here is the documentation of map::operator[] from the current C++ standard...

23.4.4.3 Map Element Access

T& operator[](const key_type& x);

1. Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.

2. Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.

3. Returns: A reference to the mapped_type corresponding to x in *this.

4. Complexity: logarithmic.

T& operator[](key_type&& x);

1. Effects: If there is no key equivalent to x in the map, inserts value_type(std::move(x), T()) into the map.

2. Requires: mapped_type shall be DefaultConstructible.

3. Returns: A reference to the mapped_type corresponding to x in *this.

4. Complexity: logarithmic.

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