Are temporary objects in C++ const indeed?

Question

I always believed that temporary objects in C++ are automatically considered as const by the compiler. But recently I experienced that the following example of code:

Answer 1

Temporary objects can be const, but they don't have to be.

((string const)"hello").append(" world"); // error!

It allows for various things. Consider

struct bitref {
  int index;
  bitref &operator=(bool value); // non-const!
};

struct bitset {
  int flags;
  // returns a bitref temporary that's associated with the bit
  // at index 'index'. 
  bitref operator[](int index); 
  // ...
};

You could do

bitset b;
b[1] = true; // change second bit to 1

This is what's done by the std::bitset<> template.