map insert函数返回值的含义

#include <map>
#include <iostream>

int main(){

std::map< int,int > ll;
ll.insert( std::pair< int,int >(1,2) );

std::pair< std::map< int,int >::iterator,bool > ret;
ret=ll.insert( std::pair< int,int >(2,3) );

if( ret.second ){
std::cout<<"成功插入"<<std::endl;
std::cout<< "map(" <<ret.first->first<<","<<ret.first->second<< ")" << std::endl;
}
else{
std::cout<<"失败"<<std::endl;
}

return 0;
}