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Re: Passing an array as argument to a function

Calin Negru29-Mar-20 2:58

Calin Negru

29-Mar-20 2:58

thanks Greg for that tip

Re: Passing an array as argument to a function

leon de boer29-Mar-20 4:11

leon de boer

29-Mar-20 4:11

Under C any pointer is already a pointer to an array it's built into the language along with pointer arithmetic ... you need to just learn that.

Literally declare any pointer of anything lets do a float
float* p;
now you can access it as an array
p[0] = 5.0;
p[100] = 10.0;
It will crash because the pointer isn't really to any memory but it makes the point the pointer is already a pointer to an array

There are no exceptions to the rule it doesn't matter if the pointer is to a fundamental type or struct .... so I don't get how you could ever forget that. In the C community the [] use is rare because it's two extra characters to type. It also has implication when declaring variables because it puts that array on the stack not on data memory or constant memory (rodata) if it determines its a constant. So if you get into the habit of using that form you can get some undesirable things happen.

Personally you are learning and I would learn to live without it and just learn them as you will most often see them written.

The topic is well covered in dummies guide to C but you will note the last statement
How to Use Arrays and Functions Together in C Programming - dummies[^]

In vino veritas

Re: Passing an array as argument to a function

Greg Utas29-Mar-20 5:54

Greg Utas

29-Mar-20 5:54

I made a mistake. See Richard's post below.

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Re: Passing an array as argument to a function

Calin Negru29-Mar-20 6:07

Calin Negru

29-Mar-20 6:07

noticed

modified 29-Mar-20 12:16pm.

Re: Passing an array as argument to a function

Richard MacCutchan29-Mar-20 4:19

Richard MacCutchan

29-Mar-20 4:19

Greg Utas wrote:
char* somedata[]

That's an array of pointers.

Re: Passing an array as argument to a function

leon de boer29-Mar-20 4:21

leon de boer

29-Mar-20 4:21

good pick didn't even notice the * because that form is so foreign to me.

In vino veritas

Re: Passing an array as argument to a function

Greg Utas29-Mar-20 5:53

Greg Utas

29-Mar-20 5:53

Right you are! I forget to remove the *.

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Re: Passing an array as argument to a function

Richard MacCutchan29-Mar-20 6:55

Richard MacCutchan

29-Mar-20 6:55

Why not leave the * but remove the []? The parameter is a pointer to an array, not an actual array.

Re: Passing an array as argument to a function

Greg Utas29-Mar-20 7:07

Greg Utas

29-Mar-20 7:07

I use brackets to indicate that the underlying is an array, not a pointer to a single char. Maybe it's because I was a latecomer to C++ and never used C idioms, another one being if(p), for which I write if(p != nullptr).

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Re: Passing an array as argument to a function

Richard MacCutchan29-Mar-20 7:20

Richard MacCutchan

29-Mar-20 7:20

A pointer rarely means a pointer to a single item, it always indicates a pointer to a set of items. If you want to pass a single int, char etc, then why use a pointer?

I would agree with you on using the if(p != nullptr) construct, it makes it much clearer.

Re: Passing an array as argument to a function

Greg Utas29-Mar-20 7:50

Greg Utas

29-Mar-20 7:50

Richard MacCutchan wrote:
If you want to pass a single int, char etc, then why use a pointer?

Because it might inadvertently be nullptr, and I find this defensive code jarring:

void f(type& t)
{
    if(&t ! nullptr)...
}

The optimize-everything crowd won't agree, but in my opinion code that invokes the above with a null reference should suffer a SIGSEGV before the function is called. But since that's not the case...

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Re: Passing an array as argument to a function

Richard MacCutchan29-Mar-20 21:07

Richard MacCutchan

29-Mar-20 21:07

I don't think a reference can ever be null.

Re: Passing an array as argument to a function

Greg Utas30-Mar-20 2:40

Greg Utas

30-Mar-20 2:40

It's true that C++ has no explicit notion of a null reference. But if you run this

void test(int& i)
{
   if(i == 1)
      std::cout << i << '\n';
}

int main(int argc, char* argv[])
{
   int* pi = nullptr;
   test(*pi);
}

it will SIGSEGV on the line if(i == 1). That's in a VS2017 debug build.

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Re: Passing an array as argument to a function

Richard MacCutchan30-Mar-20 3:28

Richard MacCutchan

30-Mar-20 3:28

That is interesting. It should really crash at the test(*pi); line, since it is trying to dereference a null pointer. I would also suggest the the compiler should recognise that pi is a pointer and not a reference.

Re: Passing an array as argument to a function

Greg Utas30-Mar-20 3:35

Greg Utas

30-Mar-20 3:35

I agree that it should crash there. But I've never seen it work that way, though for most of my career I worked in a language where it would have crashed there.

It's not unusual to dereference a pointer (pi) and pass it to an argument that wants a reference.

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Re: Passing an array as argument to a function

Richard MacCutchan30-Mar-20 4:04

Richard MacCutchan

30-Mar-20 4:04

Greg Utas wrote:
It's not unusual ...

Interesting, but not something I have ever done. I had (naively) assumed that the whole point of references was to avoid this very trap. Incidentally I tried it in g++ as well and the gave a SEGV.

Re: Passing an array as argument to a function

Greg Utas30-Mar-20 4:11

Greg Utas

30-Mar-20 4:11

Where did it die in g++? Before or after calling the function?

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Re: Passing an array as argument to a function

Richard MacCutchan30-Mar-20 4:24

Richard MacCutchan

30-Mar-20 4:24

Same as in Windows, on the if statement in test function.

Re: Passing an array as argument to a function

k505430-Mar-20 4:58

k5054

30-Mar-20 4:58

Ditto for clang 9.0.1 (Fedora fc31)

Keep Calm and Carry On

Re: Passing an array as argument to a function

k505429-Mar-20 7:52

k5054

29-Mar-20 7:52

Richard MacCutchan wrote:
f you want to pass a single int, char etc, then why use a pointer?

On occasion you want an "out" or sentinel parameter, so in those cases you have to use a pointer (or a reference if using C++).
There's lots of cases where you might have a pointer to a single struct that you either want to fill in, or avoid copying the whole thing to the stack. For the latter, of course, you'd mark it as const.

Keep Calm and Carry On

Re: Passing an array as argument to a function

Calin Negru29-Mar-20 8:11

Calin Negru

29-Mar-20 8:11

how do you declare and access an array of pointers? if you want to cycle through pointers of same type in a for loop.

int * somedata[] = new int *  [5]; ??

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modified 29-Mar-20 14:42pm.

Re: Passing an array as argument to a function

k505429-Mar-20 9:42

k5054

29-Mar-20 9:42

In that case you'd use another level of indirection: e.g.

C++

#include <iostream>

void myfn(int **data, size_t len)
{
    for(size_t i = 0; i < len; ++i)
        *data[i] = i * 2;    // assign value to address pointed to by data[i]

    //  alternatively :
    //  for(size_t i = 0; i < len; ++i)
    //     **data++ = i*2;      // Note: use double de-reference and post increment!
}

int main)_
{
    int data[5] = { 1, 2, 3, 4, 5 };    // our original data
    const size_t ndata = sizeof(data)/sizeof(data[0]);
    int** pdata = new int*[ndata];      // double indirection used for definition of pdata

    // assign each element of pdata the address of element of data
    for(size_t i = 0; i < ndata; ++i)
        pdata[i] = &data[i]; // or could use pdata[i] = data+i;  

    std::cout << "Before:\n";
    for(size_t i = 0; i < ndata; ++i)
        std::cout <^lt; *pdata[i] << std::endl;

    myfn(pdata, 5);

    std::cout <*lt; "\nAfter:\n";
    for(size_t i = 0; i < ndata; ++i)
        std::cout << *pdata[i] << std::endl;

    delete[] pdata;

    return 0;
}

Keep Calm and Carry On

modified 29-Mar-20 17:49pm.

Re: Passing an array as argument to a function

Calin Negru30-Mar-20 22:30

Calin Negru

30-Mar-20 22:30

thanks k5054

Re: Passing an array as argument to a function

Calin Negru31-Mar-20 21:36

Calin Negru

31-Mar-20 21:36

This array of pointers thing is above trivial. Here is my old code:

CUSTOMVERTEX* ScreenLetters;
	ScreenTextBuffers[0]->Lock( 0, 0, (void**)&ScreenLetters, 0 );
	 int LetterVertexDataincrement = 0;
	 int letterwidth = 12;
	 int letterheight = 12;
	 
	 for(int ii = 0; ii < ScreenLetterGroups[0].height;ii++)
	 {
		 for(int i = 0; i < ScreenLetterGroups[0].width;i++)
		{
			//. .
		    //_ .
			ScreenLetters[LetterVertexDataincrement].position.x = i * letterwidth + ScreenLetterGroups[0].x;
			ScreenLetters[LetterVertexDataincrement].position.y = ii * letterheight + ScreenLetterGroups[0].y;
		 ScreenLetters[LetterVertexDataincrement].position.z = 20;
		 ScreenLetters[LetterVertexDataincrement].color = 0xffffffff;
// ... filling the array ...

}
}

ScreenTextBuffers[0]->Unlock();

I have this piece of code repeating for every ScreenTextBuffers element. I want to place it in a `for` loop so what I`m doing is:

CUSTOMVERTEX ** ScreenLettersP_s = new CUSTOMVERTEX* [NumberOfTextBuffers];
	 int LetterVertexDataincrement = 0;
	 int letterwidth = 12;
	 int letterheight = 12;
	for(int iii =0; iii < NumberOfTextBuffers; iii++)
	{
		ScreenTextBuffers[iii]->Lock( 0, 0, (void**)&ScreenLettersP_s[iii], 0 );
		
	 
	 for(int ii = 0; ii < ScreenLetterGroups[iii].height;ii++)
	 {
		 for(int i = 0; i < ScreenLetterGroups[iii].width;i++)
		{
			//. .
		    //_ .

		 *ScreenLettersP_s[LetterVertexDataincrement]->position.x = i * letterwidth + ScreenLetterGroups[iii].x; // `illegal indirection`
		 *ScreenLettersP_s[LetterVertexDataincrement]->position.y = ii * letterheight + ScreenLetterGroups[iii].y;
		 *ScreenLettersP_s[LetterVertexDataincrement]->position.z = 20;
		 *ScreenLettersP_s[LetterVertexDataincrement]->color = 0xffffffff;

// ... filling the array ...

}
}
 ScreenTextBuffers[i]->Unlock();
}

but it doesn`t compile, I`m getting an `illegal indirection` error

struct CUSTOMVERTEX
{
    D3DXVECTOR3 position; // The position
    D3DCOLOR    color;    // The color
    FLOAT       tu, tv;   // The texture coordinates
};

modified 1-Apr-20 6:03am.

Re: Passing an array as argument to a function

Richard MacCutchan31-Mar-20 23:02

Richard MacCutchan

31-Mar-20 23:02

Arrays of pointers are just as trivial as arrays of anything, if you understand how to address them.

fearless_ wrote:
I`m getting an `illegal indirection` error

Since we cannot see your screen we also cannot guess where that occurs. Please format your code properly and explain exactly where the error occurs.

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