Compilation failure associated with vector of const std::string

Question

Please can someone explain why the following code doesn't compile using clang 3.5.

The error reported is 'No viable overloaded '=' in algorithm.'

std::vector<const std::string> m_messages;
std::vector<const std::string>::iterator iter;
...

if (iter != m_messages.end())
{
    m_messages.erase(iter);      // compilation error
}

If I declare m_messages as: std::vector<std::string> m_messages; then it compiles OK.

Also, what is the difference between:

std::vector<const std::string> m_messages;

and

std::vector<std::string> m_messages;

TIA.

Answer 1

To erase an element, the right hand side elements have to be relocated (shifted to the left).

Since your strings are const, the old element cannot be overwritten (via the = operator) hence the error.

Answer 2

Does that mean it makes no sense to have a vector of const strings if the elements can be removed? Yes at least what the standard says

23.3.7.5 vector modifiers [vector.modifiers]

iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last);

Effects: Invalidates iterators and references at or after the point of the erase.

Complexity: The destructor of T is called the number of times equal to the number of the elements erased, but the move assignment operator of T is called the number of times equal to the number of elements in the vector after the erased elements.

Throws: Nothing unless an exception is thrown by the copy constructor, move constructor, assignment operator, or move assignment operator of T.