Programming Forums > Application Development > C++

deanosrs · Jan 7th, 2008, 2:08 PM

Hi,
I'm having a problem with c++ - I have an object that needs to have as one of its variables an array - however, I won't know how big this array needs to be until instances of the class are constructed.

So where I'm tripping up is over my confusion of arrays as pointers etc - probably quite a common problem! This is the relevant code I have in my header file (I've generalised it so when someone gives me a solution I'll understand better and not hassle you guys again):

 c++ Syntax (Toggle Plain Text) 
...
class image
{
public:
...
	int arrayName[];
...
};
...
...
class image
{
public:
...
	int arrayName[];
...
};
...

And then in the .cpp file I have something like:

 c++ Syntax (Toggle Plain Text) 
image::image(string input)
{
	// read in file containing image data determined by string
	// now put this data into array
	int newArray[(imgWidth * imgHeight)];
 
	// now need to make the object's variable point to this local function variable
	???!!
}
image::image(string input)
{
	// read in file containing image data determined by string
	// now put this data into array
	int newArray[(imgWidth * imgHeight)];

	// now need to make the object's variable point to this local function variable
	???!!
}

Thanks very much in advance for any help anyone can offer.

~deano

Narue · Jan 7th, 2008, 2:23 PM

>So where I'm tripping up is over my confusion of arrays as pointers etc
An array isn't a pointer and a pointer isn't an array. Anyway, C++ doesn't support variable length arrays. You have to provide a compile-time constant or don't use an array.

One of the alternatives is to simulate an array by allocating memory to a pointer:

 c++ Syntax (Toggle Plain Text) 
int *arrayName;
 
// In the constructor:
arrayName = new int[imgWidth * imgHeight];
 
// In the destructor:
delete [] arrayName;
int *arrayName;

// In the constructor:
arrayName = new int[imgWidth * imgHeight];

// In the destructor:
delete [] arrayName;

A better alternative is to use the std::vector class instead. It handles the memory for you.

mbd · Jan 7th, 2008, 2:57 PM

it seems as though you are actually tripping up on what happens to memory allocated on the stack after the function it was allocated for returns.

Jan 7th, 2008, 2:08 PM	#1
deanosrs Programmer Join Date: Apr 2006 Location: Bristol, UK Posts: 31 Rep Power: 0	How to make array with undefined size as object variable Hi, I'm having a problem with c++ - I have an object that needs to have as one of its variables an array - however, I won't know how big this array needs to be until instances of the class are constructed. So where I'm tripping up is over my confusion of arrays as pointers etc - probably quite a common problem! This is the relevant code I have in my header file (I've generalised it so when someone gives me a solution I'll understand better and not hassle you guys again): c++ Syntax (Toggle Plain Text) ... class image { public: ... int arrayName[]; ... }; ... ... class image { public: ... int arrayName[]; ... }; ... And then in the .cpp file I have something like: c++ Syntax (Toggle Plain Text) image::image(string input) { // read in file containing image data determined by string // now put this data into array int newArray[(imgWidth * imgHeight)]; // now need to make the object's variable point to this local function variable ???!! } image::image(string input) { // read in file containing image data determined by string // now put this data into array int newArray[(imgWidth * imgHeight)]; // now need to make the object's variable point to this local function variable ???!! } Thanks very much in advance for any help anyone can offer. ~deano

Jan 7th, 2008, 2:23 PM	#2
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	Re: How to make array with undefined size as object variable >So where I'm tripping up is over my confusion of arrays as pointers etc An array isn't a pointer and a pointer isn't an array. Anyway, C++ doesn't support variable length arrays. You have to provide a compile-time constant or don't use an array. One of the alternatives is to simulate an array by allocating memory to a pointer: c++ Syntax (Toggle Plain Text) int arrayName; // In the constructor: arrayName = new int[imgWidth imgHeight]; // In the destructor: delete [] arrayName; int arrayName; // In the constructor: arrayName = new int[imgWidth imgHeight]; // In the destructor: delete [] arrayName; A better alternative is to use the std::vector class instead. It handles the memory for you. __________________ Even if the voices aren't real, they have some pretty good ideas.

Jan 7th, 2008, 2:57 PM	#3
mbd Programmer Join Date: Nov 2007 Posts: 86 Rep Power: 2	Re: How to make array with undefined size as object variable it seems as though you are actually tripping up on what happens to memory allocated on the stack after the function it was allocated for returns.

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How to make array with undefined size as object variable