SDL_SetClipping

I don’t understand how SDL_Setclipping() works.
I tried following source code:
"
SDL_Surface* object;
SDL_Rect rect;

object = SDL_LoadBMP("./pic.bmp");
rect.x = 0;
rect.y = 0;

SDL_SetClipping(object,100,500,300,100);
while (x!=640)
{
rect.x ++;
SDL_Blit(…);
SDL_Flip(screen);
}"

This code doesn’t work. I wanted to move the object smooth in and out of
this Clipping Rectangle but everytime the object goes out of the rectangle
whith just one Pixel it disapears.

What have i done wrong ?–
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