Passing by Reference. Simple question.

Hey guys,

I'm stuck. Like everyone else posting in here I guess :)

Anyway, what I'm trying to do is really simple. I have a struct, and a function to modify any struct of the same type passed as param. That's the issue here... I know that I should be using pointers, but my app just refuses to compile!

I'm using the default C compiler that comes with Dev C++ 4.9.9.0.
Here goes my code:

Code:

---

#include <stdio.h>
#include <conio.h>

struct Test
{
    int a;
    int b; 
};   

int main()
{

    //create a test structure and assign values
    struct Test myTest;
    myTest.a = 10;
    myTest.b = -50;
   
    //Modify the test structure
    ModifyStruct(&myTest);
   
    //Display values
    printf("%d\t%d", myTest.a, myTest.b);
    getch();

    //End program
    return 0;
}   

//Function to modify a structure
void ModifyStruct(struct Test * someTest)
{
    someTest.a = 10;
    someTest.b = 10;
}

---

This is insane! I get a few errors, but it all seems to be because of:

32 C:\...\test.c request for member `a' in something not a structure or union
33 C:\...\test.c request for member `b' in something not a structure or union


Help!
I would appreciate an example with reference parameter passing with structs :)

-Rowan.

Code:

---

void ModifyStruct(struct Test * someTest)
{
    someTest.a = 10;
    someTest.b = 10;
}

---

The red part declares your argument as a pointer.

The blue code tries to use the pointer as a struct.

You need to change it to use '->' instead of '.'

Or you could use the more obscure (*sometest).a etc. [But this is more typing and less readable, in my opinion].

--
Mats

Hey there, matsp!

Thank you! I tried the code. It works!!!

Thank you so much man :)

For anybody who'd like to see the full code, here goes:

Code:

---

#include <stdio.h>
#include <conio.h>

struct Test
{
    int a;
    int b; 
};   

int main()
{

    //create a test structure and assign values
    struct Test myTest;
    myTest.a = 10;
    myTest.b = -50;
   
    //Modify the test structure
    ModifyStruct(&myTest);
   
    //Display values
    printf("%d\t%d", myTest.a, myTest.b);
    getch();

    //End program
    return 0;
}   

//Function to modify a structure
void ModifyStruct(struct Test * someTest)
{
    someTest->a = 10;
    someTest->b = 10;
}

---

And yes, I think -> looks better :)

-Rowan.

You're passing by a pointer, not reference. References are only possible in C++.
Remember that you need to add * to dereference a pointer (and obj->a is basically another syntax for (*obj).a).

It's really a matter of terminology.

Passing by a pointer is a way of "passing by reference" as opposed to "pass by value" because you can modify what the pointer points to. You can't modify the pointer itself, though, as that value is passed by value. If you wanted to modify the address of a pointer you'd have to pass a pointer to a pointer.

Pass by reference is a general-use term that can apply to any programming language and does not refer to the C++ data type reference.

Well, I'd rather say pass by pointer because that's what you're doing in sense of the language. It's called a pointer, so it would be pass by pointer.
Ah well.

Technically, C has only one passing mechanism: By value. If you want to use a "reference", the way to do that is to pass the value of a pointer to the data you want to refer to.

You may say that arrays are passed by reference, but really, technically, they are just passing the value of the pointer to the first element.

I agree that "pass by pointer" makes somewhat more sense than "pass by reference", especially since the latter IS a term used in C++.

--
Mats