What does putting a '|' on function parameter do?

Question

I am learning how to write SDL program in C++, and I came across this code:

SDL_Renderer *ren = 
    SDL_CreateRenderer(win, -1, SDL_RENDERER_ACCELERATED | SDL_R         


        

Answer 1

These are flags which you can set. In this instance | refers to the bitwise operator.

In your example, this conveniently allows you to combine multiple flags through a single parameter.

Suppose the two flags have the following values:

SDL_RENDERER_SOFTWARE = 1 // Binary 0001
SDL_RENDERER_ACCELERATED = 2 // Binary 0010
SDL_RENDERER_PRESENTVSYNC = 4 // Binary 0100

A logic bitwise OR of the two, would leave you with the value 6 for the flag. We can easily determine from this value which flags have been set using bitwise AND.:

flag = SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC // flag = 6 (0110)
flag & SDL_RENDERER_SOFTWARE == 0 // Not set
flag & SDL_RENDERER_ACCELERATED == 2 // Set
flag & SDL_RENDERER_PRESENTVSYNC == 4 // Set

Note that it's important here for the flags to be powers of two, to ensure all flag combinations result in a unique value.