Converting infix expression to postfix expression (including some functions)

[C++]

I have found numerous examples involving only arithmetic operations but mine requires to deal with functions - min(x, y), max(x, y), sqrt(x), abs(x), fact(x). What I don't understand is how my program can interpret those functions and set the precedence for them. Examples: 4 + 5 -> 4 5 + abs(-5)...