Explanation of alternative for variable length array

[dinners]

Hi there, I am very new to C++ and I want to create an array of variable length.
I tried this:

int corners;
	cout << "Please enter the number of corners: ";
	cin >> corners;
	int x[corners];

but it came up with errors:
error C2057: expected constant expression
error C2466: cannot allocate an array of constant size 0
error C2133: 'x' : unknown size

After some research I found this as a solution:

int corners;
	cout << "Please enter the number of corners: ";
	cin >> corners;
	int * const x = (int*)_alloca(corners * sizeof(int));

Whilst this does work, I was hoping someone could explain exactly what it meant/was doing.

I am using Visual C++ Express 2008.

Thanks very much for any help! :)

[arkoenig]

Don't do it -- it's a local extension and not part of C++. Do this instead:

int corners;
cout << "Please enter the number of corners: ";	cin >> corners;
vector<int> x(corners);    // Note: parentheses, not square brackets

You have to say

#include <vector>

for this to work.

[dinners]

Thanks arkoenig!

I can see why this is a better solution. I have looked up the vector function and I can see why it would be good to do what I want. However, I can't fully get my brain around what the statment

vector<int> x(corners)

is doing.

Sorry for being a bit slow!

[arkoenig]

It's defining a variable named x of type vector<int> and using the value of corners to initialize it. When you use an integer to initialize a vector, that integer determines the number of elements the vector has.

[dinners]

Ah ok, thanks so much for your help!

[arkoenig]

By the way, many people who use vectors also use the push_back function. For example:

vector<int> x;
int n;
while (cin >> n)
    x.push_back(n);

Calling x.push_back(n) pushes a copy of n onto the back of x. That is, it increases the size of x by 1 and uses n to initialize the newly created element.

This technique makes it unnecessary to know in advance how many elements the vector will have.

[Fbody]

A vector is based on a C++ system called a "template". You don't have to worry about exactly what a template is and how they work at this point, I'm sure you'll learn soon enough. In absolute simplest terms, a template allows you to define a class or function one (1) time and make it work for virtually any dataType without having to re-write it.

A vector's declaration includes this dataType as what is known as a "template argument" (the part between '<' and '>'). Thus, the general form of the declaration of a vector can be described as:

vector< elementDataType > vectorName(vectorSize)

"I want to create a vector of elementDataType objects. I want to call the vector "vectorName" and it will have "vectorSize" elements in it."

EDIT:
Oops... What arkoenig said too...
I really didn't want to mention it to prevent confusion, but there are actually several ways of defining a vector. The methods mentioned in the thread are probably the 2 most-common ways.

[dinners]

Thanks to both of you for your help. The push.back function would actually be very handy for me and would make me code much neater and more logical!