在STL映射中，是否更好地使用map :: insert比[]？ [英] In STL maps, is it better to use map::insert than []?

问题描述

过去，我曾与一位同事讨论如何在STL中插入值地图 。我喜欢
map [key] = value;
，因为它感觉很自然，很容易阅读，而他更喜欢
map.insert（std :: make_pair（key，value））

我刚刚问他，我们都不记得原因为什么插入更好，但我相信它不只是一个风格的偏好，而是有一个技术原因，如效率。 SGI STL参考简单地说严格来说，此成员函数不必要：它仅仅为了方便而存在。

任何人都可以告诉我这个理由，或者我只是在做梦吗？

方案

当你写

  map [key] = value; 
  

无法判断您是否已取代 > 键键键，或者创建一个新的键 value 。

map :: insert（） 只会创建：

 使用std :: cout;使用std :: endl; 
 typedef std :: map< int，std :: string> MyMap; 
 MyMap地图; 
 // ... 
 std :: pair< MyMap :: iterator，bool> res = map.insert（std :: make_pair（key，value））; 
 if（！res.second）{
 cout<< key<<键<< already exists
<< 具有值< （res.first） - >第二<< endl; 
} else {
 cout<< 创建密钥<<键<< 具有值<值<< endl; 
} 
  

对于大多数应用程序，我通常不在乎创建或替换，所以我使用更容易阅读 map [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))

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."

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

解决方案

When you write

map[key] = 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() will only create:

using std::cout; using std::endl;
typedef std::map<int, std::string> MyMap;
MyMap map;
// ...
std::pair<MyMap::iterator, bool> res = map.insert(std::make_pair(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;
}

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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