Non-static const member, can’t use default assignment operator

You have a case like this:

struct sample {
    int const a; // const!

    sample(int a):a(a) { }
};

Now, you use that in some context that requires sample to be assignable – possible in a container (like a map, vector or something else). This will fail, because the implicitly defined copy assignment operator does something along this line:

// pseudo code, for illustration
a = other.a;

But a is const!. You have to make it non-const. It doesn’t hurt because as long as you don’t change it, it’s still logically const 🙂 You could fix the problem by introducing a suitable operator= too, making the compiler not define one implicitly. But that’s bad because you will not be able to change your const member. Thus, having an operator=, but still not assignable! (because the copy and the assigned value are not identical!):

struct sample {
    int const a; // const!

    sample(int a):a(a) { }

    // bad!
    sample & operator=(sample const&) { }
};

However in your case, the apparent problem apparently lies within std::pair<A, B>. Remember that a std::map is sorted on the keys it contains. Because of that, you cannot change its keys, because that could easily render the state of a map invalid. Because of that, the following holds:

typedef std::map<A, B> map;
map::value_type <=> std::pair<A const, B>

That is, it forbids changing its keys that it contains! So if you do

*mymap.begin() = make_pair(anotherKey, anotherValue);

The map throws an error at you, because in the pair of some value stored in the map, the ::first member has a const qualified type!