What exactly is a renderer in SDL2? - c++

Question

I dont exactly understand what renderer is. Can I have multiple renderers or is there always just one?
For example, how can I draw a rectangle with a certain color on a background with a different color using a renderer?
I believe the answer lies in the functions SDL_RenderDrawRect() and SDL_RenderFillRect(). Am I right?
I know how surfaces and bliting works but I dont know what exactly the renderer symbolizes.
If someone could show me how to draw a rectangle, I think i will understand how renderers work.
So far I have this:
#include <SDL.h>
int main(int argc, char* argv[]) {
//Initialization
SDL_Init(SDL_INIT_EVERYTHING);
//Window
SDL_Window *MainWindow = SDL_CreateWindow("My Game Window",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
640, 480,
SDL_WINDOW_SHOWN
);
//Renderer
SDL_Renderer *Background = SDL_CreateRenderer(MainWindow, -1, 0);
SDL_SetRenderDrawColor(Background, 255, 255, 255, 255);
SDL_RenderClear(Background);
SDL_Delay(3000);
//Clean up
SDL_DestroyWindow(MainWindow);
SDL_Quit();
return 0;
}

Answer 1

for the first part of your question see this SO question.
as to why your code doesnt do much:
you are correct that you need to use either SDL_RenderDrawRect(), or SDL_RenderFillRect().
SDL_RenderDrawRect will draw an unfilled rectangle. SDL_RenderFillRect will be filled (hopefully that is obvious).
With SDL_renderer you need to call SDL_RenderPresent to copy the "scene" to the screen.
...
//Renderer
SDL_Renderer* renderer = SDL_CreateRenderer(MainWindow, -1, 0);
SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
SDL_RenderClear(renderer); // fill the scene with white
SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255); // the rect color (solid red)
SDL_Rect rect(0, 0, 100, 50); // the rectangle
SDL_RenderFillRect(renderer, &rect);
SDL_RenderPresent(renderer); // copy to screen
SDL_Delay(3000);
...