Question

I have an accessor function that returns a const reference to a type (std::map) … …

myMap_t const& getMap() const {return paramMap;} 

The type has an overloaded [] operator. What is the syntax to then use the [] operator directly from the getter function in a way like the following, but that actually works.

parameter = contextObj.getMap()[key];

Error message is:

context.cpp:35: error: passing   
  'const std::map<
     std::basic_string<char, std::char_traits<char>, std::allocator<char> >, 
     float, 
     std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, 
     std::allocator<std::pair<
       const std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
       float> > >'
as 'this' argument of 
  '_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&)  
with 
  _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
  _Tp = float, 
  _Compare = std::less<std::basic_string<char, std::char_traits<char>,  td::allocator<char> > >, 
  _Alloc = std::allocator<std::pair<
    const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, 
    float> >]' 
discards qualifiers

Answer 1

The problem is that operator[] in a map is a mutating operation, and you cannot call it on a const reference to a map. The reason that it is mutating is that it must return a reference to a value, and for that, if the key was not already present in the container, it will insert a new default constructed value addressed by that key and return the reference to it.

If you have a const reference, and you want to determine whether an element is present (or access it), you must use std::map<>::find that will return an iterator. If the element is not present the value of the iterator would be m.end()