map中[]运算符和find的区别

此文章转载自：https://blog.csdn.net/weixin_39831546/article/details/78314476

map的下标运算符[]的作用是：将关键码作为下标去执行查找，并返回对应的值；如果不存在这个关键码，就将一个具有该关键码和值类型的默认值的项插入这个map。

map m;
m["first"] = "hello";
m["second"] = "world";

cout<<"[]查找之前"<::iterator it = m.begin();
for(;it!=m.end();++it)
{
    cout<first<<" "<second<first<<" "<second<

//operator []的源码
mapped_type& operator[](const key_type& _Keyval)
{ // find element matching _Keyval or insert with default mapped
iterator _Where = this->lower_bound(_Keyval);
if (_Where == this->end()
|| this->comp(_Keyval, this->_Key(_Where._Mynode())))
_Where = this->insert(_Where,
value_type(_Keyval, mapped_type()));
return ((*_Where).second);
}

map的find函数：用关键码执行查找，找到了返回该位置的迭代器；如果不存在这个关键码，就返回尾迭代器。

map f;
f.insert(make_pair("first","hello"));
f.insert(make_pair("second","world"));
cout<<"find查找之前"<::iterator it = f.find("third");
if(it != f.end())
{
    cout<second<first<<" "<second<

//再看看find的源码
const_iterator find(const key_type& _Keyval) const
{ // find an element in nonmutable sequence that matches _Keyval
const_iterator _Where = lower_bound(_Keyval);
return (_Where == end()
|| _DEBUG_LT_PRED(this->comp,
_Keyval, _Key(_Where._Mynode()))
? end() : _Where);
}