Question

The SDL documentation say that SDL_BlitSurface() returns -1 when it is not successful, but i can not find why it would fail.

Here is my source code:

main.cpp

#include <iostream>
#include <string>
#include <SDL/SDL.h>
#include <SDL/SDL_image.h>
#include "window.cpp"
#include "player.cpp"

int init();
void quit();

int main(int argc,char *args[])
{
    if (init() == -1) 
    {
        std::cerr << "Error:init()" << std::endl;
        return -1;
    }
    window main_window("Juego","img/bg.png",800,600,32);
    player player1(500,500,0,0,5,5,"img/square.png");
    while (!main_window.close)
    {
        main_window.handle_events();
        if ( main_window.draw_background() != true)
        {
            std::cerr << "ERROR->main_window.draw_background()" << std::endl;
            main_window.close = true;
        }
        player1.update(main_window.screen);
        SDL_Flip(main_window.screen);
        SDL_Delay(60/1000);
    }
    quit();
    return 0;
}

int init()
{   
    if (SDL_Init(SDL_INIT_EVERYTHING) == -1)
    {   
        std::cerr << "Error while initialising SDL" << std::endl; 
        return -1;
    }
    return 0;
}

void quit()
{
    SDL_Quit();
}

window.cpp

class window
{
    private:
        void load_image(std::string source,SDL_Surface *destination);
        SDL_Surface *window_bg;
        SDL_Event event_queue;
        SDL_Rect window_rect;
    public:
        window(std::string SCREEN_TITLE,std::string SCREEN_BG,int sw,int sh,int sbpp);
        void handle_events();
        bool draw_background();
        SDL_Surface *screen;
        bool close;
};

window::window(std::string SCREEN_TITLE,std::string SCREEN_BG,int sw,int sh,int sbpp)
{
    window_bg = NULL;
    screen = NULL;
    close = false;
    window_rect.x = 0;
    window_rect.y = 0;
    window_rect.w = sw;
    window_rect.h = sh;
    screen = SDL_SetVideoMode(sw,sh,sbpp,SDL_SWSURFACE);
    std::cout << "Screen created." << std::endl;
    SDL_WM_SetCaption(SCREEN_TITLE.c_str(),NULL);
    std::cout << "Window title: " << SCREEN_TITLE << std::endl;
    load_image(SCREEN_BG,window_bg);
}

void window::handle_events()
{
    while (SDL_PollEvent(&event_queue))
    {
        if (event_queue.type == SDL_QUIT)
        {    
            close = true;
        }
    }
}

void window::load_image(std::string source,SDL_Surface *destination)
{
    SDL_Surface *tmp_img = IMG_Load(source.c_str());
    if (tmp_img != NULL)
    {
        if ((destination = SDL_DisplayFormat(tmp_img)) == NULL)
        {
            std::cerr << "ERROR->SDL_DisplayFormat()" << std::endl;
        }
        std::cout << source << " Loaded." << std::endl;
        SDL_FreeSurface(tmp_img);
    }
    else
    {    
        std::cerr << "Could not load: " << source << std::endl;
    }
}

bool window::draw_background()
{
    int error_check = SDL_BlitSurface(window_bg,&window_rect,screen,NULL);
    if (error_check != 0)
    {
        std::cerr << "SDL_BlitSurface() == " << error_check << std::endl;
        return false;
    }
    return true;
}

The error is in window.cpp (i think):

bool window::draw_background()
{
    int error_check = SDL_BlitSurface(window_bg,NULL,screen,NULL);
    if (error_check != 0)
    {
        std::cerr << "SDL_BlitSurface() == " << error_check << std::endl;
        return false;
    }
    return true;
}

The output of my program is:

Screen created.
Window title: Juego
img/bg.png Loaded.
img/square.png Loaded
SDL_BlitSurface() == -1
ERROR->main_window.draw_background()

Answer 1

load_image(SCREEN_BG,window_bg); in the window constructor loads into a temporary SDL_Surface and never populates the destination, which I assume is an output parameter. Then when draw_background() uses window_bg, it’s still NULL.