Question

This method:

void LRU::displayQueue() const
{
   for(iter = m_buffer.begin(); iter != m_buffer.end(); ++iter)
      std::cout << (*iter) << " ";
   std:: cout << std::endl;
}

results in the following error:

lru.cpp:58: error: passing 'const std::_Deque_iterator<int, const int&, const int*>' as    'this' argument of 'std::_Deque_iterator<int, const int&, const int*>& std::_Deque_iterator<int, const int&, const int*>::operator=(const std::_Deque_iterator<int, const int&, const int*>&)' discards qualifiers

m_buffer and iter are declared in my header file, where the buffer is declared as a deque of type int and iter is a constant iterator:

// ...

std::deque<int> m_buffer;
std::deque<int>::const_iterator iter;

// ...

Removing the const in the displayQueue method will eliminate the compiler error, but since this function shouldn’t modify any data in the deque, I want to make this explicit by keeping my code “const-correct”. Why would this result in an error, when my iterator is a const_iterator?

Answer 1

You’re modifying the object by setting the value of your const iterator member variable, thus invalidating the const restriction of your function. Use a local variable.

A const_iterator only means the iterator cannot modify the sequence, but you’re not modifying the sequence, you modifying the member variable iterator itself.