Question

in c++

class bar
{
    int i;
    char b;
    float d;
};

void foo ( bar arg );
void foo ( bar &arg );
void foo ( bar *arg );

this is a sample class/struct and functions
i have some Qs

• what’s the difference between 1st and 2nd way of passing the argument in ‘asm’, size, speed ?
• how the arguments are passed to the functions foo in each case ( in case of pointer i know the pointer is pushed on the stack )
• when passing arguments, in terms of efficiency at ( speed, size, preferability ) which is better ?
• what’s the intel ‘asm’ syntax that corresponds each of the ways of passing arguments ?

i know what most say about “it doesn’t matter on modern compilers and CPUs” but what if we’re talking about Old CPUs or compilers?

thanks in advance

Answer 1

The pointer and the reference methods should be quite comparable (both in speed, memory usage and generated code).

Passing a class directly forces the compiler to duplicate memory and put a copy of the bar object on the stack. What’s worse, in C++ there are all sort of nasty bits (the default copy constructor and whatnot) associated with this.

In C I always use (possibly const) pointers. In C++ you should likely use references.