CODEWITHSUNDEEP
c++11stl
Jai Prabhu
Jai Prabhu

Reputation: 207

C++: Error retrieving STL map from another map

I have the following piece of code - 

#include <map>

using Type1 = std::map<std::string, unsigned long long>;
using Type2 = std::map<std::string, Type1>;

class T
{
   private:
      Type2 mymap;

   public:
      const Type1& get(const std::string& key) const;
};

const Type1& T::get(const std::string& key) const
{
   return mymap[key];
}

int main(void)
{
}

This does not compile and the compiler complains -

maps.cpp: In member function ‘const Type1& T::get(const string&)
const’: maps.cpp:17:20: error: passing ‘const Type2 {aka const
std::map<std::basic_string<char>, std::map<std::basic_string<char>,
long long unsigned int> >}’ as ‘this’ argument of ‘std::map<_Key, _Tp,
_Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string<char>; _Tp = std::map<std::basic_string<char>, long
long unsigned int>; _Compare = std::less<std::basic_string<char> >;
_Alloc = std::allocator<std::pair<const std::basic_string<char>, std::map<std::basic_string<char>, long long unsigned int> > >;
std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type =
std::map<std::basic_string<char>, long long unsigned int>;
std::map<_Key, _Tp, _Compare, _Alloc>::key_type =
std::basic_string<char>]’ discards qualifiers [-fpermissive]
return mymap[key];
                ^

I need help understanding this error. From what I can tell I am not modifying "this" in the get function.

Thanks!

Upvotes: 0

Views: 58

Answers (1)

P0W
P0W

Reputation: 47794

The errors comes from the fact that std::map::operator[] can inserts the key if the it does not exist.

You could do a std::map::find first as a check.

Upvotes: 2

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