Thread: I need an explanation about references

Originally Posted by 4c0

When i call foo function, does it push its arguments onto the stack? What is the layout of a stack frame? What is the difference between pass by reference and pass by a pointer?

You could look at the stack frame:

Code:

#0 00000000	foo(aa=@0x28fefc: 1, bb=@0x28fef8: 1) (C:\Users\Josh2\Documents\bar\bar2.cpp:7)
#1 004013B0	main() (C:\Users\Josh2\Documents\bar\bar2.cpp:18)

Clearly something is being passed in as an argument. It is functionally equivalent to a pointer, if that isn't confusing. References are a way to pass an object to a function without worrying about copying it or NULL references from a pointer.

I read somewhere that there are no references to references, but i compile this:

That is reference assignment, which you can do. It creates two references to the same object. In the code you posted, you have the reference to int b, and reference to int c both referencing the integer a. It's not exactly the same as a pointer to a pointer. In that sort of situation, you have a pointer value, which at that memory location also stores a pointer value.

Originally Posted by 4c0

When i call foo function, does it push its arguments onto the stack? What is the layout of a stack frame?

Does it matter? All you need to know is what it does.
These things are not really defined in the standard and will vary between different platforms and compilers, so there is no single answer to this question. So again, what does it matter?

What is the difference between pass by reference and pass by a pointer?

A reference is simple an alias (a different name) for a variable you pass in. So you are referring to a variable you passed in, but you've named it differently.
A pointer passes the address of a variable, which you can then use to write and read to/from the address specified in the variable (and obviously change the address (value) stored inside the variable, too).

I read somewhere that there are no references to references...

References to references are illegal. Consider this:

Code:

int main()
{
	int x;
	int&&& z = x;
}

This gives a compile error:
error C2529: 'z' : reference to reference is illegal

As to why there are 3 &, it's because && is also a type of reference (r-value reference); thus, it's legal, while &&& is not.

Originally Posted by 4c0

I read somewhere that there are no references to references, but i compile this:

Code:

#include<iostream>

using namespace std;


int main(){

int a(0);
int& b=a;
int& c=b;


return 0;
}

That is not an example of references to references. b and c are both alternative names for (or references to) the variable a. b is a reference to a. c is a reference to the same variable as b (i.e. to a). It is not a reference to a reference.

Originally Posted by Elysia

As to why there are 3 &, it's because && is also a type of reference (r-value reference); thus, it's legal, while &&& is not.

r-value references are only valid in C++-11. Not in earlier versions of C++.

Originally Posted by whiteflags

https://www.google.com/search?q=practical+knowledge

Unless you are optimizing for a system, that's not necessary.

*shrug*
I can be wrong. It doesn't matter to me.
But I'm also not going to believe it has any practical purpose unless you are dealing with such a low level unless someone can prove otherwise.

I only recently started learning C++ however from what I know up till now I tend to look at C++ "passed-by-reference" as basically const pointers. They both accomplish the same thing but one requires less code. Doesn't mean pointers cannot accomplish the same thing

Code:

  1 #include<iostream>
  2 using namespace std;
  3 
  4 void foo(int& a,int& b){
  5 
  6     a=0;
  7     b=0;
  8 }
  9 
 10 void foo_ptrs(int* a, int * b){
 11 
 12    *a = 0;
 13    *b = 0;
 14 }
 15 
 16 int main(void){
 17 
 18    int a = 1, b = 1;
 19 
 20    foo(a,b);
 21    cout << "using foo() a = " << a <<" b ="<< b <<endl;
 22 
 23    foo_ptrs(&a,&b);
 24    cout <<"using foo_ptrs() a = " << a << " b = " << b << endl;
 25 
 26    return 0;
 27 }

A reference does the same thing as a pointer in that it allows you to access the address of a variable and change it from within function scope. However
It is supposed to be "safter" in that you cannot change the value it is pointing to. Whereas in C you can. C++ folks will argue its merits but for now
I simply use it and see it as

Code:

const int* ptr

or

Code:

int const *ptr

.

Always opened to hear more from those who've been working with it longer though. I'm still a C fan boy at heart

Originally Posted by hex_dump

It is supposed to be "safter" in that you cannot change the value it is pointing to.

And it is illegal to obtain a "null reference", whereas it is legal to pass a null pointer, hence the function must check for it or just declare the behaviour to be undefined.

Originally Posted by hex_dump

I simply use it and see it as

Code:

const int* ptr

or

Code:

int const *ptr

I think you mean:

Code:

int * const ptr