Try assignments.
I just tried that, and I think it's working. Could you explain why you have to assign some, and other's is like in parenthesis?
like for example
sf::RectangleShape myRectangleShape;
myRectangleShape.setFillColor(sf::Color::Blue); <---- parenthesis
sf::FloatRect myFloatRect;
myFloatRect.top = 20; <----- assignment