How to operator[] access element in a const map in C++?

How to operator[] access element in a const map in C++? For example, the compiler will report error on this piece of code:

#include <iostream>
#include <string>
#include <map>

std::int64_t count(const std::map<std::string, std::int64_t>& map)
{
  return map["one"] + map["two"];
}

int main ()
{

  std::map<std::string, std::int64_t> map = {
    {"one", 1},
    {"two", 2}
  };

  std::cout << count(map) << std::endl;

  return 0;
}

g++, using command

$ g++ --std=c++11 main.cc -o a

will report

main.cc: In function ‘int64_t count(const std::map<std::basic_string<char>, long int>&)’:
main.cc:7:19: error: passing ‘const std::map<std::basic_string<char>, long int>’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::basic_string<char>; _Tp = long int; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, long int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = long int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]’ discards qualifiers [-fpermissive]
   return map["one"] + map["two"];
                   ^
main.cc:7:32: error: passing ‘const std::map<std::basic_string<char>, long int>’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::basic_string<char>; _Tp = long int; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, long int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = long int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]’ discards qualifiers [-fpermissive]
   return map["one"] + map["two"];
                                ^
asked Dec 29, 2017 by anonymous

1 Answer

To access the data of a const map in C++, use the at() member function,

const mapped_type & at (const key_type &__k) const 

instead of operator[]. The reason why operator[] can not be used for a const map is because that, if the key from operator[] does not exist, a pair with that key is created using default values and then returned, which will actually change the map.

Note that at() may throw std::out_of_range exception if no such data is present.

For the code in the question, to make it work, change

std::int64_t count(const std::map<std::string, std::int64_t>& map)
{
  return map["one"] + map["two"];
}

to

std::int64_t count(const std::map<std::string, std::int64_t>& map)
{
  return map.at("one") + map.at("two");
}
answered Jan 7 by Eric Z Ma (44,200 points)

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