Overloaded [] operator on template class in C++ with const / nonconst versions

Question

Whew, that was a long title.

Here's my problem. I've got a template class in C++ and I'm overloading the [] operator. I have both a const and a non-const version, with the non-const version returning by reference so that items in the class can be changed as so:

myobject[1] = myvalue;

This all works until I use a boolean as the template parameter. Here's a full example that shows the error:

#include <string>
#include <vector>
using namespace std;

template <class T>
class MyClass
{
    private:
        vector<T> _items;

    public:

        void add(T item)
        {
            _items.push_back(item); 
        }

        const T operator[](int idx) const
        {
            return _items[idx];
        }

        T& operator[](int idx)
        {
            return _items[idx];
        }

};


int main(int argc, char** argv)
{
    MyClass<string> Test1;      //  Works
    Test1.add("hi");
    Test1.add("how are");
    Test1[1] = "you?";


    MyClass<int> Test2;         //  Also works
    Test2.add(1);
    Test2.add(2);
    Test2[1] = 3;


    MyClass<bool> Test3;        // Works up until...
    Test3.add(true);
    Test3.add(true);
    Test3[1] = false;           // ...this point. :(

    return 0;
}

The error is a compiler error and the message is:

error: invalid initialization of non-const reference of type ‘bool&’ from a temporary of type ‘std::_Bit_reference’

I've read up and found that STL uses some temporary data types, but I don't understand why it works with everything except a bool.

Any help on this would be appreciated.

Answer 1

Because vector<bool> is specialized in STL, and does not actually meet the requirements of a standard container.

Herb Sutter talks about it more in a GOTW article: http://www.gotw.ca/gotw/050.htm