SFML将精灵放在中心

Question

I have been trying to fix this problem for about an hour or more... But couldn't find any usefull answers. I am trying to place a sprite on the center of the window, but it shows up on TOP_LEFT. Here is the constructor for my class and as you can see I am dividing surface.width and surface.height by 2

Spaceship::Spaceship(sf::RenderWindow& game_window){
    auto surface = game_window.getSize();
    signed int ss_x = surface.x/2;
    signed int ss_y = surface.y/2;

    int ss_width = 128;
    int ss_height = 128;
    int ss_radius = ss_width/2;
}
  ///////////////////////////////////////////
 // For displaying the sprite on window   //
///////////////////////////////////////////
void Spaceship::drawsprite(sf::RenderWindow& game_window){
    sf::Texture ship;
    if (!ship.loadFromFile(resourcePath() + "space-shuttle-64.png")) {
        return EXIT_FAILURE;
    }
    sf::Sprite ss_sprite(ship);
    ss_sprite.setPosition(ss_x, ss_y);
    game_window.draw(ss_sprite);
}

I have also tried with:

   auto surface = game_window.RenderWindow::getSize();
    signed int ss_x = surface.x/2;
    signed int ss_y = surface.y/2;

but that didn't help either.

UPDATE:

I have tried to print the variables which are defined inside the constructor and I got 0 on all of them. So my problems seems to be the accessing problem. but there were no errors or warnings that have told me that.

UPDATE 2:

this is the header file:

#ifndef Spaceship_hpp
#define Spaceship_hpp
#include <iostream>
#include <SFML/Audio.hpp>
#include <SFML/Graphics.hpp>
#include <stdio.h>

using namespace std;

class Spaceship {
public:
    Spaceship();
    Spaceship(sf::RenderWindow&);
    ~Spaceship();
     void moveship(char);
     void drawsprite(sf::RenderWindow&);

private:
    signed int ss_x, ss_y;
    unsigned int ss_speed;
    int ss_width, ss_height, ss_radius;

};

#endif /* Spaceship_hpp */

Answer 1

You are not initialising your attributes in your constructor correctly.

Spaceship::Spaceship(sf::RenderWindow& game_window){
    auto surface = sf::VideoMode::getDesktopMode();
    signed int ss_x = surface.width/2;
    signed int ss_y = surface.height/2;

    int ss_width = 128;
    int ss_height = 128;
    int ss_radius = ss_width/2;
}

should be

Spaceship::Spaceship(sf::RenderWindow& game_window){
    auto surface = sf::VideoMode::getDesktopMode();
    ss_x = surface.width/2;
    ss_y = surface.height/2;

    ss_width = 128;
    ss_height = 128;
    ss_radius = ss_width/2;
}

Declaring variables in the body of the class means they are seen globally by the class, if you redeclare a variable in the constructor, it will take over the role from the global variable. This is called variable shadowing . All modifications to the variable will work, but once you leave the scope of your constructor/function/method, then you lose the information since your attribute variable wasn't modified.

More on scopes: http://en.cppreference.com/w/cpp/language/scope

More on variable shadowing: https://en.wikipedia.org/wiki/Variable_shadowing?oldformat=true