std::map - 元素访问无异常且无插入

问题描述：

我有一个使用 std::map 的循环模式.

I have a recurrent pattern with the use of std::map.

我只想在键存在时检索值，否则我不想插入元素.目前我正在使用 count(key) 或 find(key) (哪个更好?从文档中，复杂性似乎相同)，如果它们返回一个我访问地图的正值.但是我想避免在地图上使用两个操作.类似的东西:

I want to retrieve the value only when the key is present, otherwise I don't want to insert element. Currently I'm using count(key) or find(key) (which one is better? from the documentation the complexity seems to be the same) and if them returns a positive value that I access the map. However I would like to avoid the use of two operations on the map. Something like:

map<string, int> myMap;
int returnvalue;
boole result = myMap.get("key1",returnValue)
if(result){
  \\ use returnValue
}

阅读关于 cplusplus.com 的 std::map 文档，我发现了两个函数用于访问地图元素:

Reading the std::map documentation on cplusplus.com I found two functions for accessing map elements:

at():如果密钥不存在则抛出异常礼物
[]:如果键不存在

at(): which throws an excpetion if the key is not present
[]: which insert a new value if the key is not present

没有一个能满足我的需要.

None of them satisfy my necessity.

答

使用地图::查找:

auto it = myMap.find(key);

if (it != myMap.end())
{
    // use it->second
}
else
{
    // not found
}

这部分很简单.更难的问题是当你想查找一个元素是否存在并在它存在时返回它，否则在那个键上插入一个新元素，所有这一切都不需要搜索地图两次.为此，您需要使用 lower_bound 后跟提示插入.

This part was easy. The harder problem is when you want to look up if an element exists and return it if it does, but otherwise insert a new element at that key, all without searching the map twice. For that you need to use lower_bound followed by hinted insertion.

std::map - 元素访问无异常且无插入

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