Multidimensional array calculating

Question

I've got an array of arrays of TCHAR, where I store file extension.

TCHAR *extens[] = { L".*", L".txt", L".bat" };

In order to go through it, I'm calculating it's length.

int extCount = sizeof(extens) / sizeof(TCHAR);

But for some reason the extCount's value is 2. I think the problem is because this is wrong calculation method, but then, how to count the number of elements ("words") in this array correctly?

UPD: I'am passing this array to function:

void func(TCHAR *path, TCHAR **names, TCHAR **extensions);

When i'am calculating this array lenght outside function it show correct number, but inside it always workis wrong (returns 2 or 1).

UPD2: I tried to redeclare array like this:

TCHAR *extens[] = { L".txt", L".bat", L".txt", NULL };

And now inside function i'am doing something like that:

TCHAR **p = extensions;
    int extCount = 0;
    while (*p != NULL)
    {
        extCount++;
        *p++;
    }

    extCount = cnt;
    wsprintf(temp, L"%d", cnt);
    MessageBox(NULL, temp, temp, MB_OK);

It works, but looks like its not so effective, because of walking two arrays, isn't it?

What are you expecting ? You put one string in your array, so the length will be 1. — ElderBug, Jul 27 '15 at 08:30
@ElderBug, no its always 1, no matter how many elements are there — Ivan Petrov, Jul 27 '15 at 08:32
@Ivan Petrov `extCount` is 3 for me. Have you re-built the program? — LogicStuff, Jul 27 '15 at 08:37
You should use `sizeof(extens) / sizeof(*extens)`, it's less error prone. — ElderBug, Jul 27 '15 at 08:38
@LogicStuff, VS2013 automatically build new application when start debugging. — Ivan Petrov, Jul 27 '15 at 08:42
@LogicStuff Does it? The operands of sizeof aren't evaluated, right? — JorenHeit, Jul 27 '15 at 08:42
@IvanPetrov `TCHAR**` is not an array, so you can't calculate an array length on it. You have to pass the length with an additional parameter. — ElderBug, Jul 27 '15 at 08:50
Or preferably you'd have your function templated on array-size and be done with it :-) — JorenHeit, Jul 27 '15 at 08:54
@IvanPetrov I think you'd be better off using `string`s and `vector`s rather than messing around with all those pointers. If you can't use those for whatever reason, and all these arrays are constructed at compile-time, you should make that function a template and let your compiler figure out the sizes. — JorenHeit, Jul 27 '15 at 08:57
@JorenHeit , i need to finish this using in that way. Also maybe i can use pointer and go through array and count number of elements? — Ivan Petrov, Jul 27 '15 at 08:59
@IvanPetrov See my answer, you can use the templatized version which will deduce N and M as the correct sizes without you having to bother. — JorenHeit, Jul 27 '15 at 09:07
A side note: you are using TCHAR incorrectly. There are two correct ways to use TCHAR: the old, clumsy one is with the _T macro (`_T("x")` not `L"x"`), and the new, preferred one is **never**. — n. m. could be an AI, Jul 27 '15 at 09:18
Use `char` when you need it and `wchar_t` when you need it. There is zero sense in switching between them at compile time. — n. m. could be an AI, Jul 27 '15 at 09:21
And naturally you don't want to use built-in arrays or pointers either. We have std:string and std::vector. — n. m. could be an AI, Jul 27 '15 at 09:31

score 1 · Answer 1 · answered Jul 27 '15 at 08:25

1

TCHAR *extens[] is an array of pointers of type TCHAR. And the size of such an array will be array_length * sizeof(pointer)).

Note: sizeof(pointer) on a system will be same for all datatypes.

answered Jul 27 '15 at 08:25

paper.plane

1,201
10
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So i need to get the array length ( to know how many elements are there). – Ivan Petrov Jul 27 '15 at 08:29
"sizeof(pointer) on a system will be same for all datatypes". AFAIK this is not guaranteed by the C++ standard. In particular, it is possible that `sizeof(char*)>sizeof(otherdatatype*)` and TCHAR can stand for `char` (if the OP fixes the program to use _T() correctly). – n. m. could be an AI Jul 28 '15 at 12:26

score 1 · Answer 2 · answered Jul 27 '15 at 08:38

1

You have an array of TCHAR*.

To get the length of the following array:

TCHAR *extens[] = { L".*", L".txt", L".bat" };

You need to use:

sizeof(extens) / sizeof(TCHAR*)

answered Jul 27 '15 at 08:38

Nasser Al-Shawwa

3,573
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JorenHeit · Answer 3 · 2015-07-27T09:26:46.317

1

First of all, you have an array of pointers so you need

extCount = sizeof(extens) / sizeof(TCHAR*);

to calculate its size. However, this assumes that extens is still of an array-type. Once you pass it to a function expecting a TCHAR**, the array will decay to a pointer and its size information will be lost.

I think your best option would be to rewrite this in terms of std::string and std::vector. This is C++ so you might as well use its facilities. If this is not possible for any reason, and the arrays are known at compile time, you could templatize the function on array-sizes:

template <size_t N, size_t M>  
void func(TCHAR *path, TCHAR *(&names)[N], TCHAR *(&extensions)[M]);

The syntax is a bit messy maybe. For example, TCHAR *(&names)[N] is read as: "names is a reference to an array of N pointers to TCHAR". Here, the size N is deduced by the compiler as long as you don't let the array decay to a plain pointer.

edited Jul 27 '15 at 09:26

answered Jul 27 '15 at 09:06

JorenHeit

3,877
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The problem is that arrays could be dynamically changed during execution. – Ivan Petrov Jul 27 '15 at 09:08
If that is the case, you can't use C style arrays at all like you do in your question. If you need dynamically sized arrays, you definately need `std::vector`. – JorenHeit Jul 27 '15 at 09:13
@IvanPetrov you are still using static arrays so my answer still holds. The way it is now, your arrays cannot change size. – JorenHeit Jul 27 '15 at 09:23

Multidimensional array calculating

3 Answers3