Parameters by Reference

Hi all,

I'll start by saying sorry for my ignorance - I am 100% a hobby C programmer for a text-based game. I am a professional C# developer (sorry) so most of my problems are around pointers - and this is one of them.

I have two functions, headed:

Code:

void	one_hit					args((CHAR_DATA *ch, CHAR_DATA *victim, int dt, int wpn));
int		get_one_hit_damage		args((CHAR_DATA *ch, CHAR_DATA *victim, int dt, int wpn));

the first is small so I'll post it:

Code:

void one_hit(CHAR_DATA *ch, CHAR_DATA *victim, int dt, int wpn)
{
	int dam = get_one_hit_damage(ch, victim, dt, wpn);
    if(dam == -1) return;
    damage(ch, victim, NULL, dam, dt, wpn);
    tail_chain();
    return;
}

the second is huge but to cut a long story short, it returns an integer and, importantly, changes the value of dt.

Having read all I can about passing parameters by reference in C, I have come to this conclusion (echo'd in the header, or without - same error):

Code:

int get_one_hit_damage(CHAR_DATA *ch, CHAR_DATA *victim, int &dt, int wpn)
{
    ...
    dt = 1;
    return 100;
    ...
}

... I get the impression that using the & here infront of the dt parameter should make it a reference parameter. However when I compile I get...

Code:

fight.c:611: error: expected ';', ',' or ')' before '&' token

I've tried putting the same & in the function header but it doesn't like that either.

I'm sure this is simple but I just don't know how to work it...

Can anyone help?

Cheers,

Dom

You cannot pass by reference in C. What you remember is C++. In C++, there is indeed the option to use & to pass by reference. In C, there are only pointers.
Now, is there a reason you're using C instead of C++?

Haha that is an excellent question!

I don't know is the honest answer. The game has been running for decades which is the most likely reason - I inherited it and have been working with it for the last 3 years and this kind of thing is always getting me.

Is it a difficult thing to do to move it to C++? There is a /lot/ of code...

C++ is backwards compatible. Some code will break (such as conversions from void*), but mostly it should be fine.
It should allow you to use C++ features in the future to interact with the legacy C code.

Don't confuse pointer usage, like int *, with obtaining the address of an object using & operator.

Code:

int *ip;   /* ip is a variable of type pointer to integer */
int i;     /* i is a variable of type integer */
ip = &i;   /* now ip points to i */

The OP was not confusing the address-of operator with pointers. He/she was confusing the C++ syntax of passing by reference with C.

Ok excellent thank you... that's very interesting.... one more question then, if you would - I am not a *nix user so the environment is much as it was and I use a makefile to compile my application.

Presumably to upgrade to C++ all I really need to do is compile the code with a C++ compiler instead of a C one and fix any errors that arise.

My current makefile looks like this

Code:

CC      = gcc
TARGET  = dw 
DEBUG   = -g 
DEFINES = -DNOCRYPT 
C_FLAGS = -O -Wall $(DEBUG) $(DEFINES)
L_FLAGS = $(DEBUG)

O_FILES = act_comm.o act_info.o act_move.o act_obj.o act_wiz.o bit.o board.o clan.o cledit.o comm.o const.o db.o dice.o fight.o file.o handler.o hedit.o hunt.o interp.o magic.o mob_prog.o olc.o quest.o sedit.o spec_mob.o spec_obj.o ssm.o string.o trap.o update.o

$(TARGET): $(O_FILES)
	$(CC) $(L_FLAGS) -o $@ $^

%.o: %.c merc.h bit.h
	$(CC) $(C_FLAGS) -c $<

clean:
	rm -f *.o $(TARGET)

There is likely to be a default C++ compiler on the server - it's used for lots of stuff - do you have any idea what it may be called and what would need to change in the makefile to compile?

Thank you so much for your advice!

Dom

IBTD, simply because how the function header is coded

Code:

int get_one_hit_damage(CHAR_DATA *ch, CHAR_DATA *victim, int &dt, int wpn)

tho' apparently the op does fine with the CHAR_DATA ones!

Originally Posted by dominicshaw

Ok excellent thank you... that's very interesting.... one more question then, if you would - I am not a *nix user so the environment is much as it was and I use a makefile to compile my application.

Presumably to upgrade to C++ all I really need to do is compile the code with a C++ compiler instead of a C one and fix any errors that arise.

My current makefile looks like this

There is likely to be a default C++ compiler on the server - it's used for lots of stuff - do you have any idea what it may be called and what would need to change in the makefile to compile?

Thank you so much for your advice!

Dom

It would probably be called g++ since you seem to be using GCC.

Originally Posted by itCbitC

IBTD, simply because how the function header is coded

Code:

int get_one_hit_damage(CHAR_DATA *ch, CHAR_DATA *victim, int &dt, int wpn)

Exactly. C++ syntax.

No she is quite right I'm afraid - I read that putting & in front of it would make it a reference parameter on what I now realise to have been a C++ website.

I did experiment with getting a pointer to the dt parameter and passing that in as a pointer but because of everything else going on in the page (other calls using a constant to get_one_hit_damage) - i couldn't get anywhere...

I did something like

Code:

int *p_dt;
p_dt = &dt;
int dam = get_one_hit_dam(ch, victim, p_dt)

but i got nowhere.

how do you do this kind of thing in C?

Basically the two functions used to do the same thing but one just returned a number and the other actually did something with the number - I'm jsut trying to tidy it up so I dont have to update both functions for every change...

Code:

void foo(int* p)
{
    *p = 10;
}

int main()
{
    int x;
    foo(&x);
    // x == 10.
}

Fair enough - right - well I want to move with the times - or a few decades behind them at least so I am going to go ahead and try to upgrade. I have started and got over 3000 errors which is a great beginning.

I've got that down a lot already but I am slightly (very) confused by something which I think you have already hinted at - something to do with void* - a generic pointer, as I understand it.

This is an area of the application I've skillfully avoided for the last 3 years, and it is to do with memory allocation. The problem is in this code which, I take it, allocates a load of memory for a load of objects (structures) to load into memory...

Code:

    OBJ_DATA **prgpstrShow;
    int *prgnShow;
    //get count
    prgpstrShow = alloc_mem(count * sizeof(OBJ_DATA *));
    prgnShow = alloc_mem(count * sizeof(int));

...and...

Code:

void *alloc_mem(int sMem)
{
    void *pMem;
    int iList;

    for (iList = 0; iList < MAX_MEM_LIST; iList++)
    {
        if (sMem <= rgSizeList[iList])
            break;
    }

    if (iList == MAX_MEM_LIST)
    {
        bug("Alloc_mem: size %d too large.", sMem);
        exit(1);
    }

    if (!rgFreeList[iList])
    {
        pMem    = alloc_perm(rgSizeList[iList]);
    }

    else
    {
        pMem              = rgFreeList[iList];
        rgFreeList[iList] = * ((void **)rgFreeList[iList]);
    }

    return pMem;
}

I know this is now completely separate but if you could point me in the right direction it would save me a lot of time! I think it'll be the main hurdle as I can see a lot of these types of errors on the horizon.

Thanks,

Dom

Change

Code:

    //get count
    prgpstrShow = alloc_mem(count * sizeof(OBJ_DATA *));
    prgnShow = alloc_mem(count * sizeof(int));

to

Code:

    //get count
    prgpstrShow = reinterpret_cast<OBJ_DATA **>( alloc_mem(count * sizeof(OBJ_DATA *)) );
    prgnShow = reinterpret_cast<int*>( alloc_mem(count * sizeof(int)) );

C++ does not allow implicit conversion from void* to T*, where T is any type except void. You must do an explicit cast.
In C++ (not C code compiled as C++), casts are usually avoided due to C++'s type system which typically works on the fact that the type can be known at all times.