C++ “error: passing 'const std::map >' as 'this' argument of …”

Question

With the following code (excerpted for brevity):

color.h:

class color {
public:
    color();

    enum colorType {
        black, blue, green, cyan,          


        

Answer 1

First : the map map<int, string> colors must be a map from cColorType to string instead of int :

map<cColorType, string> colors

Second : As some people already answered : map::operator[]() is not const. The reason for that is that this operator returns a reference, which allows you to modify its value.

Let me suggest the following solution : You can create a second private attribute : the color in string format. Therefore you'll have 2 Get functions (one for each type of color), and one Set function (which will modify the 2 color attributes).

Answer 2

The key is near the end: "discards qualifiers". getColorText is a const member function, so colors is const. But map::operator[]() is not const.

Answer 3

string color::getColorText() const {
    return colors[cColortype];
}

The issue is that you've marked the function as const. The operator[] on std::map is marked as non-const, and cannot be used in a const function like this. You need to manually use std::map::find (or other mechanism) to search for the input type and handle the case where it's not found.

If you're using C++11, you can instead use std::map::at, which IS allowed to be used on a constant map, and throws an exception if the requested element is not present.