问题描述

前段时间，我和一位同事讨论了如何在 STL 中插入值 地图.我更喜欢 map[key] = value; 因为它感觉自然且易于阅读，而他更喜欢 map.insert(std::make_pair(key, value)).

A while ago, I had a discussion with a colleague about how to insert values in STL maps. I preferred map[key] = value; because it feels natural and is clear to read whereas he preferred map.insert(std::make_pair(key, value)).

我刚问过他，我们都不记得插入更好的原因，但我确信这不仅仅是风格偏好，而是效率等技术原因.SGI STL 参考 简单地说:严格来说，这个成员函数是不必要的: 它的存在只是为了方便."

I just asked him and neither of us can remember the reason why insert is better, but I am sure it was not just a style preference rather there was a technical reason such as efficiency. The SGI STL reference simply says: "Strictly speaking, this member function is unnecessary: it exists only for convenience."

谁能告诉我这个原因，还是我只是梦想有一个?

Can anybody tell me that reason, or am I just dreaming that there is one?

推荐答案

写的时候

map[key] = value;

无法判断您是替换 key的value，还是创建新的key 与 value.

there's no way to tell if you replaced the value for key, or if you created a new key with value.

map::insert() 只会创建:

using std::cout; using std::endl;
typedef std::map<int, std::string> MyMap;
MyMap map;
// ...
std::pair<MyMap::iterator, bool> res = map.insert(MyMap::value_type(key,value));
if ( ! res.second ) {
    cout << "key " <<  key << " already exists "
         << " with value " << (res.first)->second << endl;
} else {
    cout << "created key " << key << " with value " << value << endl;
}

对于我的大多数应用程序，我通常不在乎我是在创建还是替换，所以我使用更易于阅读的 map[key] = value.

For most of my apps, I usually don't care if I'm creating or replacing, so I use the easier to read map[key] = value.

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