C ++中map_instance [key]与map_instance.find（key）之间的区别

Question

I have a map defined as

static map<unsigned int, deque<FOO_STRUCT*> > CV_MAP; 

and I was wondering if one method was better than the other between:

map_instance[key] vs. map_instance.find(key).

Answer 1

map_instance[key] will default construct a new value for the key key if it is not present.

As in, it will call deque<FOO_STRUCT*> 's constructor if key is not present, and return the newly constructed deque<FOO_STRUCT*> .

Using the find member function will not construct a new value for the given key-- when the key is absent, it will just return an iterator that points to CV_MAP.end() .

Use what's appropriate for your use case.

Answer 2

map_instance[key] -返回引用 （在需要之前，构造新元素） map_instance.find(key) -返回迭代器

Answer 3

map_instance[key] can be preferred where all you care is put the key with default initialization into the map, if not present, or do something with it if present.

Consider a case where you want to store count of each element in an array.

for(auto x : array){
   map_instance[x]++;  //no need to find here at first. 
}

As you can see it really simplifies your code, and gets rid of if-else clause.

for(auto x : array){
   if ( map_instance.find(key) == map_instance.end() ) {
      map_instance[x] = 0;
   }
   else{
      map_instance[x]++;
   }
}