const char* not found in map find

问题

I have a small program, I am trying to search for a particular string in the map, I get correct result if I pass static const string "us" to find but it doesn't work if I copy a string to pointer. I somehow think that it is trying to compare the address of the string that is passed. I am planning to get rid of std::string from due to some reason.

using namespace std;

static struct MarketLang {
    const char* market;
    const char* lang;
} market_lang[] = {
    {"us",  "all"},
    {"el",  "en,fr,it,de,es"},
    {"xx",  "na"},
};

class MarketLangMap {
    map<const char*, MarketLang *> table;
    public:
    MarketLangMap() {
        int TOTAL_MARKET_INFO = sizeof(market_lang)/sizeof(MarketLang);
        for (int i = 0; i < TOTAL_MARKET_INFO; i++) {
            table[market_lang[i].market] = market_lang+ i;
        }
    }

    MarketLang *operator[](const char* s) {
        if (table.find(s) != table.end()) {
            return table.find(s)->second;
        } else {
            return table.find("xx")->second;
        }
    }
};


int
main()
{

   MarketLangMap *m = new MarketLangMap();
   const char* r = "us";
   char* p = new char(10);
   strcpy(p, "us");
   std::cout<<(*m)["us"]->lang <<std::endl;``
   std::cout<<(*m)[r]->lang <<std::endl;
   std::cout<<(*m)[p]->lang <<std::endl;

}

Expected output: all all all

enter code here

Actual output: all all na

回答1:

std::map is using strict weak ordering criterion indicated by its internal comparison object, which is defaulted to the std::less.

std::less will not treat char* arguments as a string, it will just see them as a pointers, and it will just check if one pointer is less than the other.

However, you can (but I guess you should use std::string to avoid mixing C and C++), create new class for comparing char* arguments, treating them as strings:

#include <cstring>

struct ConstCharStarComparator
{
  bool operator()(const char *s1, const char *s2) const
  {
    return strcmp(s1, s2) < 0;
  }
};

map<const char*, MarketLang *, ConstCharStarComparator> table;

来源：https://stackoverflow.com/questions/16310565/const-char-not-found-in-map-find

标签

c++

string

map

char

const