Overriding sf::RectangleShape::draw

[yannik]

Hello,
I derive a class Square from RectangleShape. The Square also contains a text I want to draw. But when I override draw to draw the text, how do I also draw the rectangle? Do I have to create yet another class or is there something I'm missing? Here's the code

#include <SFML/Graphics/RectangleShape.hpp>
#include <SFML/Graphics/Text.hpp>
#include <SFML/Graphics/Font.hpp>
#include <SFML/Graphics/RenderTarget.hpp>
#include <SFML/Graphics/RenderWindow.hpp>

#include <string>

class Square : public sf::RectangleShape
{
private:
    sf::Text            mText;

public:
                                Square(const sf::Font& font, sf::Vector2f position, const std::string& s = "");

    void                        update();

private:
    void                        draw(sf::RenderTarget& target, sf::RenderStates states) const;
};

Square::Square(const sf::Font& font, sf::Vector2f position, const std::string& s)
{
        mText.setFont(font);
        mText.setString(s);
        mText.setCharacterSize(26);
        mText.setFillColor(sf::Color::White);
        mText.setPosition(sf::Vector2f(position.x + 16, position.y + 8));

        setPosition(position);
        setSize(sf::Vector2f(48, 48));
        setFillColor(sf::Color::Transparent);
        setOutlineColor(sf::Color::White);
        setOutlineThickness(1.3f);
}

void Square::draw(sf::RenderTarget& target, sf::RenderStates states) const
{
        target.draw(mText);
}

void Square::update()
{
        RectangleShape::update();
}

int main()
{
        sf::RenderWindow window(sf::VideoMode(640, 480), "");
        sf::Font font;
        font.loadFromFile("arial.ttf");
        Square square(font, sf::Vector2f(10, 10), "A");
        while(window.isOpen())
        {
                window.clear();
                square.update();
                window.draw(square);
                window.display();
        }
}
 

Thanks in advance.

[eXpl0it3r]

Your Square is not a sf::RectangleShape anymore and thus should not derive from it. Instead you can derive from sf::Drawable (and sf::Transformable if needed) and have the rectangle shape as member variable that you can then draw.

[yannik]

Hm, okay. I just thought a square is a rectangle and doesn't just own one, and should therefore be derived from it.

[eXpl0it3r]

It's the main example used to show that it violates the Liskov substitution principle.

[yannik]

Ah okay, didn't know that. Thank you

[yannik]

But then I have another question: If I now want to change the outline thickness or color of the Square, do I have to call RectangleShape::update afterwards? If so, how can I call a protected function from this context? I tried changing the color, but the Square doesn't change, and I can't call update because it's protected (which is actually why I derived from it in the first place  )

[eXpl0it3r]

The rectangle shape is now a member of your own class, so if you want to change the color or outline thickness, you simply call the public API functions of the member variable.

[Hapax]

There is a compromise here: try inheriting from sf::Shape (as sf::RectangleShape etc. do) instead of sf::Drawable and sf::Transformable directly, although you will need to write the code to determine its points etc. but you could probably look at rectangle shape's code to help.
Example of inheriting from sf::Shape to create an ellipse class is in the Shapes tutorial:
https://www.sfml-dev.org/tutorials/2.4/graphics-shape.php#custom-shape-types

[chudek123]

Hello, I would like to share with my solution which allows for inheriting from sf::RectangleShape class. I met troubles with deriving from sf::Drawable and sf::Transformable, because then I couldn't use sf::Shape::setFillColor, so I tried to inherit from sf::Shape like our friend said above. Inheriting from sf::Shape was still problematic, because I had to define points like in eclipse example, I didn't want to do this in my case of drawing rectangle with grid. Finally, inheriting from sf::RectangleShape gave me a possibility for overriding draw function:

void CustomRectangleShape::draw(sf::RenderTarget &target, sf::RenderStates states) const
{
    states.transform *= this->getTransform();
    sf::VertexArray rect_shape(sf::Quads, 4);
    sf::Color rect_color = this->getFillColor();
    rect_shape[0].position = this->getPoint(0);
    rect_shape[1].position = this->getPoint(1);
    rect_shape[2].position = this->getPoint(2);
    rect_shape[3].position = this->getPoint(3);
    rect_shape[0].color = rect_color;
    rect_shape[1].color = rect_color;
    rect_shape[2].color = rect_color;
    rect_shape[3].color = rect_color;
    target.draw(rect_shape,states);
    if(this->grid_){
        this->drawGrid(target,states);
    }
}

void CustomRectangleShape::drawGrid(sf::RenderTarget &target, sf::RenderStates states) const
{
    states.transform = grid_rec_->getTransform().Identity;
    for(int i = 0; i < grid_size_.x; i++){
        for(int j = 0; j < grid_size_.y; j++){
            grid_rec_->setPosition(this->getPosition().x + i * grid_rec_->getSize().x, this->getPosition().y + j * grid_rec_->getSize().y);
            target.draw(*grid_rec_, states);
        }
    }
}
 

I also created class fields for grid code, to be clear:

bool grid_;
sf::RectangleShape* grid_rec_;
sf::Vector2i grid_size_;

.