Question

In the following code:

class SomeClass {
    vector<int> i;
    vector<bool> b;

public:
    int& geti() {return i[0];}
    bool& getb() {return b[0];}
};

If you comment out getb(), the code compiles fine. Apparently there’s no problem returning a reference to an int that’s stored in a vector, but you can’t do it with a bool.

Why is this?

Answer 1

std::vector<bool> is “special.” It stores its elements as a bit array, meaning that the elements are not individually addressable and you cannot obtain a reference to an element.

std::vector<bool> iterators, its operator[], and its other member functions return proxy objects that provide access to the elements without requiring actual bool objects to be stored.

If you need to be able to access individual elements, consider using a std::vector<char> or defining a bool-like enumeration backed by a char (or a signed char or unsigned char, if you care about signedness).