The first one is definetely faster, as comparasion takes constant time.

But it doesn't work in the way you want. C-style strings are not strings — they're pointers to a memory where string data lies (with terminating '\0'). That's why you shouldn't write char *a = "foo", *b = "bar"; if (a < b) // ... — here pointers (that is, memory addresses) are compared, not the string themselves.

You should always use strcmp when dealing with C-strings. map<> doesn't know about it, and it will compare addresses, not strings. That can lead to a very strange behavior, don't do that, use std::string instead — it behaves more like a normal string (you can compare, modify, copy and concatenate them easily)

In pure speed, I'll vote for const char* BUT you should be careful using it

map<const char*,int> T;

int main ()
{
    T["abc"] = 1;
    cout << T["abc"];
}

• Output is 1, correct

map<const char*,int> T;

int main ()
{
    char temp[] = "abc";
    T[temp] = 1;
    cout << T["abc"];
}

• Output is 0, WRONG!

I'm not sure how it really works. In that case Lazycoder97 gave, what is happening when we have map<const char*, int> T and we write T["abc"] = 1; ? const char* is only a pointer, so I will think that some place in memory is allocated and "abc" is written here and a pointer to this place is returned, but we are not able to access it. But if it had worked in that way, cout<<T["abc"]; wouldn't have printed 1. How map knows that those "abc"'s from T["abc"] = 1 and cout<<T["abc"]; are the same "abc"'s?

I've tested on 1kk prepared random a-z strings 1kk inserts into map<char *, int, cstr_less> and map<string, int>:

struct cstr_less{ bool operator ()(char *a, char *b) const { return strcmp(a, b) < 0; } };

For strings shorter than 4 chars and longer than 15 map<char*, int> slightly outperforms map<string, int> because of absense of copying, and for 4-15 chars map<string, int> faster because of std::string cache locality for length <16 bytes in Dinkumware STL. And, of course, for >16 chars inserts into char*-map don't rely on string length (due to fixed N of random strings E(first diff char in comparator) ~ const), but into string-map do.