Using const char* as key for map/unordered_map

Question

How to create map/unordered_map that will use const char* as key directly?

If I use map<std::string,..>, then on each resolving map["abc"] = ... a new std::string object will be created. That causes a big overhead for allocating memory, creating a string object and copying the string into it.

How do I declare a map object that uses const char* directly without any overhead?

Answer 1

You can use a std::string_view:

std::map<std::string_view, int> Map;
Map["abc"] = 1; // no allocation necessary to store "abc"

It is basically a wrapper around string objects. And it's a view, which means that it doesn't own the string and thus doesn't copy and allocate memory to store a string.

Note that for small strings (and also literals), std::string doesn't allocate too due to SSO and so the overhead is minimal. Always measure before optimizing.

Answer 2

As an alternative to Rakete1111's string_view answer, you can equip your map with a suitable comparator (and hasher, for the unordered_map):

struct LesserString
{
  bool operator() (const char *lhs, const char *rhs) const
  {
    return std::strcmp(lhs, rhs) < 0;
  }
};

struct HashString
{
  std::size_t operator() (const char *arg) const
  {
    return however_you_want_to_hash_the_string();
  }
};

struct EqualString
{
  bool operator() (const char *lhs, const char *rhs) const
  {
    return !strcmp(lhs, rhs);
  }
};

std::map<const char*, WhateverValue, LesserString> m1;
std::unorderd_map<const char*, WhateverValue, HashString, EqualString> m2;