sf::Drawable and const question

[Erdrick]

Hi,

I have a class that is inheriting from sf::Drawable and in the draw() pure virtual function that I am defining for this class I decided that I wanted to loop through a vector of objects and draw some sf::Text output by re-using the same member object and just changing the string, position, and then drawing it.

I soon ran into problems because the draw() function is declared as const.

void gui_table::draw(sf::RenderTarget& target, sf::RenderStates states) const
{
        // apply the transform
        states.transform *= getTransform();

        // draw the table background
        target.draw(m_vertices);

        // draw the column names
        std::vector<column>::iterator itr;

        for (itr = m_column_headers.begin(); itr != m_column_headers.end(); ++itr) {
                m_text.setPosition(itr->rect.left + 5, itr->rect.top + 5);
                m_text.setString(itr->value);
                target.draw(m_text);
        }
}
 

Severity   Code   Description   Project   File   Line   Suppression State
Error   C2679   binary '=': no operator found which takes a right-hand operand of type 'std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<gui_table::column>>>' (or there is no acceptable conversion)   Dark Ruins   C:\Users\Erdrick\Documents\Visual Studio 2015\Projects\Dark Ruins\Dark Ruins\gui_table.cpp   64   

This got me to thinking about why draw() is declared as const?  I guess we should not expect to change things here, but I was thinking which of several approaches would be best.

1.) Should I be setting up a vector of sf::Text ahead of time to draw before i get to the draw() function?
2.) Should I make my own draw() function that is not const?
3.) Is there another recommended approach for what I am trying to do?

[Erdrick]

I would like to add I just made these two class members mutable

        mutable std::vector<column> m_column_headers;
        mutable sf::Text m_text;
 

and now the code works, but I'd still like feedback on the approach.

[Laurent]

This got me to thinking about why draw() is declared as const?

To enforce a design. Draw should always be const, and decoupled from logic update.

1.) Should I be setting up a vector of sf::Text ahead of time to draw before i get to the draw() function?

That's a valid option.

2.) Should I make my own draw() function that is not const?

Could work, but since there are ways to make it work with Drawable::draw...

3.) Is there another recommended approach for what I am trying to do?

Use a local sf::Text object. There's no reason to keep a persistant instance since you change it completely every time you use it; nothing can be cached.

I would like to add I just made these two class members mutable

You don't have to declare m_column_headers mutable, because you never modify it in the draw function. Just use a const_iterator.
For m_text, this is a good sign that it shouldn't be a member

[Erdrick]

Thanks a lot Laurent, this helps.  I'll take it from here and make it work!