Search code examples
c++sfml

SFML: image gets drawn and moves when I press a key, but doesn't move when I try to create a vector of them

The following code works... Basically when I press the spacebar, it draws a line, which continually moves in the Y direction on the screen.

// <Code that initializes window>


// set the shape
sf::CircleShape triangle(50, 3);
triangle.setPosition(300, 500);

sf::RectangleShape line;

// Start the game loop
while (window.isOpen())
{
    window.clear(sf::Color::White);
    // Process events
    sf::Event event;

    while (window.pollEvent(event))
    {
        // Close window: exit
        if (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Space) {
            line.setSize(sf::Vector2f(100,3));
            line.setRotation(90);
            line.setPosition(triangle.getPosition());
        }
    }

    // Clear screen

    window.clear();
    line.move(0, -0.1);
    window.draw(line);
    window.draw(triangle);

    // Update the window
    window.display();
}

The issue is that I can only draw one line at a time, when I want to draw multiple moving lines everytime I press the spacebar button. Therefore I've tried to create a vector of line objects. However, while drawing the lines work, the lines don't move in the Y direction like the previous code.

// set the shape
sf::CircleShape triangle(50, 3);
triangle.setPosition(300, 500);

sf::RectangleShape line;
std::vector<sf::RectangleShape> laserStack;

while (window.isOpen())
{
    window.clear(sf::Color::White);
    // Process events
    sf::Event event;

    while (window.pollEvent(event))
    {
        if (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Space) {
            line.setSize(sf::Vector2f(100,3));
            line.setRotation(90);
            line.setPosition(triangle.getPosition());
            laserStack.push_back(line);
        }
    }

    // Clear screen

    window.clear();
    
    for (sf::RectangleShape l : laserStack) {
        l.move(0, -0.3);
    }

    for (sf::RectangleShape laser : laserStack) {
        window.draw(laser);
    }
    window.draw(triangle);

    // Update the window
    window.display();
}

(Picture below shows that the lines get drawn, however they don't move).

enter image description here

I don't understand why the first code works, and the line moves upwards but the second code doesn't work... It seems like they should be equivalent?

Solution

  • When iterating over the lines, you create copies of the rectangles, and then move those copies, instead of the instances stored in the vector.

    Use reference in your for-range loops.

    for (sf::RectangleShape& l : laserStack) {
  l.move(0, -0.3);
}

for (sf::RectangleShape& laser : laserStack) {
  window.draw(laser);
}