Why should default parameters be added last in C++ functions?

Question

Why should default parameters be added last in C++ functions?

Answer 1

Jing Zeng is correct. I would like to add my remarks here. When a function is called, the arguments are pushed onto the stack from right to left. For example, let's say you have this arbitrary function.

int add(int a, int b) {
  int c;
  c = a + b;
  return c;
}

Here is the stack frame for the function:

------
  b
------
  a
------
 ret
------
  c
------

This diagram above is the stack frame for this function! As you can see, first b is pushed onto the stack, then a is pushed onto the stack. After that the function return address is pushed onto the stack. The function return address holds the location in main() from where the function was originally called, and after the function is done executing, the execution of the program goes to that function's return address. Then any local variables such as c are pushed onto the stack.

Now the key thing is that arguments are pushed onto the stack from right to left. Basically any default parameters that are supplied are literal values, which are stored in the code section of an executable. When the program execution encounters a default parameter without a corresponding argument, it pushes that literal value onto the top of the stack. Then it looks at a and pushes the argument's value onto the top of the stack. The stack pointer always points to the top of the stack, your most recently pushed variable. So any literal values you pushed onto the stack as default parameters are "behind" the stack pointer.

It was probably more efficient for the compiler to quickly first push arbitrary default literal values onto the stack first, since they're not stored in a memory location, and build up the stack quickly. Think about what would have been if the variables would have been pushed onto the stack first, and then the literals. Accessing a memory location for the CPU takes up a relatively long time compared to pulling a literal value out of a circuit or CPU register. Since it takes more time to push variables onto the stack vs. literals, the literals would have to wait, then the return address would have to wait, and the local variables would have to wait as well. It's probably not a big concern in efficiency, but that's just my theory for why default arguments are always in the rightmost positions of a function header in C++. It means that the compiler was designed as such.