Programming Forums > Application Development > C++

Soulstorm · Sep 2nd, 2007, 5:29 PM

I have a function in which i need to pass as argument an integer array. But how can I know the size of the integer array once this is inside the function?

For example, the following code:

 cpp Syntax (Toggle Plain Text) 
void foo(unsigned *p)
{
	cout << sizeof(p)/4 << '\n';
}
 
int main (int argc, const char * argv[]) {
	unsigned p[] = {1,2,3,4,5,6,7,8,9,0};
	foo(p);
    return 0;
void foo(unsigned *p)
{
	cout << sizeof(p)/4 << '\n';
}

int main (int argc, const char * argv[]) {
	unsigned p[] = {1,2,3,4,5,6,7,8,9,0};
	foo(p);
    return 0;

Will have as a result the number 1. If I try to use the sizeof operator in the main(), it will have the expected results, however, once the pointer of the array is passed as an argument, it seems I cannot use the sizeof operator.

Any ideas?

kruptof · Sep 2nd, 2007, 5:41 PM

Quote:

Originally Posted by Soulstorm

I have a function in which i need to pass as argument an integer array. But how can I know the size of the integer array once this is inside the function?

For example, the following code:

 cpp Syntax (Toggle Plain Text) 
void foo(unsigned *p)
{
	cout << sizeof(p)/4 << '\n';
}
 
int main (int argc, const char * argv[]) {
	unsigned p[] = {1,2,3,4,5,6,7,8,9,0};
	foo(p);
    return 0;
void foo(unsigned *p)
{
	cout << sizeof(p)/4 << '\n';
}

int main (int argc, const char * argv[]) {
	unsigned p[] = {1,2,3,4,5,6,7,8,9,0};
	foo(p);
    return 0;

Will have as a result the number 1. If I try to use the sizeof operator in the main(), it will have the expected results, however, once the pointer of the array is passed as an argument, it seems I cannot use the sizeof operator.

Any ideas?

Have the callee pass the size to you or loop through all the elements incrementing a variable as you go, until you reach a null (\0)

Edit: A simple search of the forum turned up a similar thread authored by me

Soulstorm · Sep 2nd, 2007, 5:50 PM

Quote:

Originally Posted by kruptof

Have the callee pass the size to you or loop through all the elements incrementing a variable as you go, until you reach a null (\0)

That won't work in my occasion, since the \0 character only goes at the end of a string. And I also think that wether an out-of-bounds bound location is null or not depends on the implementation. Anyway, seems from your thread that there isn't a way to do that.

andro · Sep 2nd, 2007, 5:52 PM

Once you reach your function, all you have is a pointer to the first element in the array.

DaWei · Sep 2nd, 2007, 6:06 PM

Let me express what Andro is saying in antoher way. You don't know the size. Wanting it to be otherwise doesn't work.

Eoin · Sep 2nd, 2007, 9:36 PM

Since the language here is C++ and not plain C I'll add that you could use something like Boost.Array here to sidestep your dilemma.

But otherwise the answer given can't be argued with. If all the function receives is a pointer then that's all it gets. The size would require at least one additional argument.

cryptod · Sep 2nd, 2007, 9:42 PM

Its impossible to do it without passing the length of thr array as a parameter.
here is a possible solution :

(Toggle Plain Text)

void foo(unsigned *p, int length)
{
    cout << length << '\n';
}

int main (int argc, const char * argv[]) {
    unsigned p[] = {1,2,3,4,5,6,7,8,9,0};
int length=sizeof(p)/sizeof(p[0]) ;
    foo(p,length);
    return 0;
}

Good luck

Soulstorm · Sep 3rd, 2007, 2:40 AM

That's what I have done until now. The only reason I am asking this is because I am making an Objective C library, filling some gaps left by Foundation Framework, but I am trying to preserve Objective C's strengths, like not knowing the size of any data structure that is passed as an argument. Anyway, if this is the only way, then that's how it's going to be done.

Thanks a lot, all of you.

andro · Sep 3rd, 2007, 3:04 AM

I'm interested, what gaps are you trying to fill?

Soulstorm · Sep 3rd, 2007, 8:13 AM

Minor gaps. Well, I am building a framework, and during development of other projects, I had encountered many problems with Cocoa. For example, NSIndexSet and its mutable counterpart could really use a hand when it comes to directly accessing unsigned integers without having to do an iteration.

Or, NSMatrix, is there any function that returns the cound of cells? Or any function that directly tells each cell to perform a selected function? Or any function that sorts cells into a Custom View using a selector and prepares them for proper displaying? (that FS2_Open Launcher sure gave me hell while writting the dynamic menus)

Also, some more support for C++ for use with OBJC++ could prove useful. Making some classes in C++ that are actually NSMutableArrays with overloaded operators would also be useful for ObjC++ programmers. Same with NSStrings. Also including automation for saving-loading files, additional routines for Cocoa Bindings when working with NSTableViews...

The project is pretty much in infant stages, since I have a lot of stuff to addn and I have some other projects to do. Most of the implemented stuff are workarounds that were implemented into my projects.

In what way are you interested?

Sep 2nd, 2007, 5:29 PM	#1
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 239 Rep Power: 3	Passing int arrays as arguments... I have a function in which i need to pass as argument an integer array. But how can I know the size of the integer array once this is inside the function? For example, the following code: cpp Syntax (Toggle Plain Text) void foo(unsigned p) { cout << sizeof(p)/4 << '\n'; } int main (int argc, const char argv[]) { unsigned p[] = {1,2,3,4,5,6,7,8,9,0}; foo(p); return 0; void foo(unsigned p) { cout << sizeof(p)/4 << '\n'; } int main (int argc, const char argv[]) { unsigned p[] = {1,2,3,4,5,6,7,8,9,0}; foo(p); return 0; Will have as a result the number 1. If I try to use the sizeof operator in the main(), it will have the expected results, however, once the pointer of the array is passed as an argument, it seems I cannot use the sizeof operator. Any ideas? __________________ Project::Soulstorm (personal homepage)

Sep 2nd, 2007, 5:52 PM	#4
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 294 Rep Power: 3	Once you reach your function, all you have is a pointer to the first element in the array. __________________ http://www.kevinherron.com/

Sep 2nd, 2007, 6:06 PM	#5
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Let me express what Andro is saying in antoher way. You don't know the size. Wanting it to be otherwise doesn't work. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Sep 2nd, 2007, 9:36 PM	#6
Eoin Hobbyist Programmer Join Date: Jun 2006 Location: Ireland Posts: 152 Rep Power: 3	Since the language here is C++ and not plain C I'll add that you could use something like Boost.Array here to sidestep your dilemma. But otherwise the answer given can't be argued with. If all the function receives is a pointer then that's all it gets. The size would require at least one additional argument. __________________ Visit my website BinaryNotions.

Sep 2nd, 2007, 9:42 PM	#7
cryptod Newbie Join Date: Sep 2007 Location: Algeria Posts: 2 Rep Power: 0	Its impossible to do it without passing the length of thr array as a parameter. here is a possible solution : (Toggle Plain Text) void foo(unsigned p, int length) { cout << length << '\n'; } int main (int argc, const char argv[]) { unsigned p[] = {1,2,3,4,5,6,7,8,9,0}; int length=sizeof(p)/sizeof(p[0]) ; foo(p,length); return 0; } void foo(unsigned p, int length) { cout << length << '\n'; } int main (int argc, const char argv[]) { unsigned p[] = {1,2,3,4,5,6,7,8,9,0}; int length=sizeof(p)/sizeof(p[0]) ; foo(p,length); return 0; } Good luck

Sep 3rd, 2007, 2:40 AM	#8
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 239 Rep Power: 3	That's what I have done until now. The only reason I am asking this is because I am making an Objective C library, filling some gaps left by Foundation Framework, but I am trying to preserve Objective C's strengths, like not knowing the size of any data structure that is passed as an argument. Anyway, if this is the only way, then that's how it's going to be done. Thanks a lot, all of you. __________________ Project::Soulstorm (personal homepage)

Sep 3rd, 2007, 3:04 AM	#9
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 294 Rep Power: 3	I'm interested, what gaps are you trying to fill? __________________ http://www.kevinherron.com/

Sep 3rd, 2007, 8:13 AM	#10
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 239 Rep Power: 3	Minor gaps. Well, I am building a framework, and during development of other projects, I had encountered many problems with Cocoa. For example, NSIndexSet and its mutable counterpart could really use a hand when it comes to directly accessing unsigned integers without having to do an iteration. Or, NSMatrix, is there any function that returns the cound of cells? Or any function that directly tells each cell to perform a selected function? Or any function that sorts cells into a Custom View using a selector and prepares them for proper displaying? (that FS2_Open Launcher sure gave me hell while writting the dynamic menus) Also, some more support for C++ for use with OBJC++ could prove useful. Making some classes in C++ that are actually NSMutableArrays with overloaded operators would also be useful for ObjC++ programmers. Same with NSStrings. Also including automation for saving-loading files, additional routines for Cocoa Bindings when working with NSTableViews... The project is pretty much in infant stages, since I have a lot of stuff to addn and I have some other projects to do. Most of the implemented stuff are workarounds that were implemented into my projects. In what way are you interested? __________________ Project::Soulstorm (personal homepage)

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