SDL2 C++ Taking a screenshot

It seems like you are mixing the rendering systems. That method will only work in the context of software rendering. For hardware rendering you should use the method SDL_RenderReadPixels(). To save the screenshot you would need a code like that:

SDL_Surface *sshot = SDL_CreateRGBSurface(0, w, h, 32, 0x00ff0000, 0x0000ff00, 0x000000ff, 0xff000000);
SDL_RenderReadPixels(renderer, NULL, SDL_PIXELFORMAT_ARGB8888, sshot->pixels, sshot->pitch);
SDL_SaveBMP(sshot, "screenshot.bmp");
SDL_FreeSurface(sshot);

Where w and h are the screen width and height (you can get these values using SDL_GetRendererOutputSize()).

In C SDL2 version 2.0.3, it works with:

fenetre=SDL_GetWindowFromId(touche.windowID); // "touche" is a   SDL_KeyboardEvent, "fenetre" is a SDL_window pointer

r_copie=SDL_GetRenderer(fenetre);

s_SnapSource=SDL_CreateRGBSurface(0,SCREEN_WIDTH,SCREEN_HEIGHT,32,
rmask,
gmask,
bmask,
amask); // s_SnapSource is a SDL_Surface pointer

SDL_LockSurface(s_SnapSource);
SDL_RenderReadPixels(r_copie,NULL,s_SnapSource->format->format,
s_SnapSource->pixels,S_SnapSource->pitch);

SDL_SaveBMP(s_SnapSource,NomFichier); // NomFichier is a char*
SDL_UnlockSurface(s_SnapSource);
SDL_FreeSurface(s_SnapSource);

/!\ ATTENTION /!\

#if SDL_BYTEORDER == SDL_BIG_ENDIAN
    Uint32 rmask = 0xff000000;
    Uint32 gmask = 0x00ff0000;
    Uint32 bmask = 0x0000ff00;
    Uint32 amask = 0x000000ff;  
#else
    Uint32 rmask = 0x000000ff;
    Uint32 gmask = 0x0000ff00;
    Uint32 bmask = 0x00ff0000;
    Uint32 amask = 0xff000000;
#endif

...must previously be set somewhere before any use of those variables of course ^^

If you want to put it in a header file make sure you put some "guards" like

#ifndef ENDIANNESS #define ENDIANNESS

...put the stuff here...

#endif

Otherwise, as said in the comments, you could have multiple definitions error when compiling :{ My bad :{

Don't hesitate to check the functions prototypes for return type and parameter(s), the comments here just giving informations, not more.