During instantiation of class member: "no operator "[]" matches these operands" when accessing std::map value with map[key]

I have been looking at this for quite some time. I don't understand what is bothering it. The minimum code that is giving me the problem is shown bellow. I comment on the line where the error occurs. Compilation errors follow after the code.

#include <map>
#include <cstdlib>
#include <cstdint>

template<std::size_t D>
class myclassA{
private:  
  std::array<std::string, D> a;
  std::map<std::string, std::size_t> a_type_num;
  std::array<std::size_t, D> a_dims;

  // used to initialize a_type_num
  inline std::map<std::string, std::size_t> return_type_dims() const{
    std::map<std::string, std::size_t> temp;
    for(auto const &s: a)
      temp.emplace(s, 0);
    for(auto const &s: a)     
      temp[s]++;
    return temp;
  };

  // used to initialize a_dims
  inline std::array<std::size_t, D> return_site_dims() const{
    std::array<std::size_t, D> temp;
    for(int i=0; i<D; i++)
      temp[i] = a_type_num[a[i]];  // ERROR!!!
    return temp;
  };

public:  
  // constructor
  myclassA(const std::array<std::string,D> &user) :  a(user)
    , a_type_num(return_type_dims())
    , a_dims(return_site_dims())
  {};
};

int main(int argc, char *argv[]){
  myclassA<4> co({"d","d","p","p"});
  return 0;
}

When compiling I get the error:

src/main.cpp(32): error: no operator "[]" matches these operands
            operand types are: const std::map<std::__cxx11::string, std::size_t, std::less<std::__cxx11::string>, std::allocator<std::pair<const std::__cxx11::string, std::size_t>>> [ const std::__cxx11::string ]
        temp[i] = a_type_num[a[i]];  
                            ^
          detected during:
            instantiation of "std::array<std::size_t={unsigned long}, D> myclassA<D>::return_site_dims() const [with D=4UL]" at line 40
            instantiation of "myclassA<D>::myclassA(const std::array<std::__cxx11::string, D> &) [with D=4UL]" at line 45

On the other hand, something like this compiles fine (I mean the code is going to do nothing, but it compiles):

#include <map>
#include <cstdlib>
#include <cstdint>

int main(int argc, char *argv[]){
  // myclassA<4> co({"d","d","p","p"});

  std::array<std::string, 10> a;
  std::map<std::string, std::size_t> a_type_num;
  std::array<std::size_t, 10> temp;
  for(int i=0; i<10; i++)
    temp[i] = a_type_num[a[i]];  

return 0;

So my question is: why is it complaining that no operator "[]" matches these operands?

The [] operator requires a std::map that is not const, because its behaviour when it does not find the key is to modify the map by inserting the key. In your code, you have the return_site_dims method which is a const method, meaning that it only has const access to all the non-static members of the myClassA class, including the a_type_num map. Therefore, this method is not able to use the [] operator on that map. To fix this, you could make return_site_dims a non-const method.