如何在 cpp 中查找 function 和 map 的字符串数据类型

Question

How find function searches the string in STL set.

See the below code snippet

#include<set>
#include<string>
#include<iostream>

using namespace std;

int main()
{
    set<string> myset{"Hello","Hi"};
    if(myset.find("Hello") != myset.end()){
        cout<<"Find"<<endl;
    }else{
        cout<<" Didn't find"<<endl;
    }
    return 0;
}

Here, I am not sure how find() method work internally, how it checks the string? Does it use like strcmp function?

Answer 1

No it uses the < operator. If a<b is false and a>b is also false then it must be that a equals b.

In fact strcmp cannot be used for std::string comparisons because strcmp treats the nul character ( '\0' ) as the end of a string, but std::string can contain nul characters.

Answer 2

std::set , whether it contains std::string or not, uses a comparator to search the set. The default comparator isstd::less , which uses the < operator for comparison.

"Hello" gets implicitly converted to a std::string , so this results in the overloaded < operator comparing std::string s, to effect your find() . Because C++ std::string can contain '\0' , this cannot use the C library's strcmp function.

In C++14 and later one can override the comparator to the std::less<void> transparent overload. This results (via a sequence of events) of employing other < overloads that compare a std::string with a const char * directly, avoiding the need to construct a temporary std::string object solely for the purpose of doing the find() . In C++20, and going forward this is implemented using the <=> overload.

Unfortunately you have to explicitly specify the std::less<void> comparator, since the default set comparator is still std::less<Key> .