CODEWITHSUNDEEP
c++
bandjalong
bandjalong

Reputation: 493

c++ vectors not like arrays

I've been getting my head round c++ for a few months, and have been directed by google to stack overflow most of the time for c++ queries. I noted frequently exhortations of the type "why don't you use a vector", and was inspired to do just that.

So, primarily to get the minor benefits of automatic memory deallocation, and being able to write typed comparison functions for sorting. I switched an array of pointers to objects to being a vector. Now I thought (incorrectly it seems) that vectors could be used more or less like arrays, and hence I initialized thusly:

cluster  clusters[LOTS];
vector<cluster *> pclust;
pclust.reserve(numClust);
for (int i=0; i<numClust; ++i)
    pclust[i] = clusters + i;

No complaints from the compiler. Then some time later I need to sort the vector on some attribute of the cluster object. So:

std::sort(pclust.begin(), pclust.end(), ClusterCompareNumSegs);

Again no problems compiling. Except the vector doesn't get sorted. It turns out that vector.size() is zero, and of course my initialization should have been 

pclust.push_back(clusters + i);

Now that's easy to fix, but I am confused, because the initial incorrect assignment was working. I successfully iterated through the vector - using the array syntax, like so:

for (clustind=0; clustind < numClust; ++clustind) {<br>
    cluster *cl = pclust[clustind];
    ...happily access *cl...

And it all worked fine. So I'm just wondering what's going on. Presumably in my initial assignments, I was trying to access elements not yet in the vector (I was trying to put them in), and the vector was throwing exceptions that I was ignoring. But nonetheless, when referencing the locations, the pointers were there. Can anyone provide enlightenment?

Upvotes: 7

Views: 463

Answers (4)

log0
log0

Reputation: 10917

cluster  clusters[LOTS];
vector<cluster *> pclust(numClust);
for (int i = 0; i < numClust; ++i)
  pclust[i] = clusters + i;

But this mean you are still using an array to store clusters. Can't you make clusters a vector ?

vector<cluster> clusters(LOTS);

Upvotes: 0

LihO
LihO

Reputation: 42083

std::vector::reserve requests that the capacity of the allocated storage space for the elements of the vector container be at least enough to hold n elements. It doesn't resize the vector, that's what std::vector::resize does.

Replace pclust.reserve(numClust); with pclust.resize(numClust);.

Alternatively you could remove pclust.reserve(numClust); call and change construction of this vector to: vector<cluster *> pclust(numClust); which yields same result.

I also suggest you to have a look at this question: std::vector reserve() and push_back() is faster than resize() and array index, why? :)

Upvotes: 5

joval
joval

Reputation: 496

operator[] used with vector returns reference to element on index-position. BUT, you haven't initialized vector with any values, so it was empty.

Altough you did a pclust.reserve(numClust), but it only tells that size of the vector will change soon and it allocates storage space without changing vector's size yet.

Upvotes: 0

Benjamin Bannier
Benjamin Bannier

Reputation: 58547

vector::reserve doesn't change the size of your vector, it still contains only the 0 elements it was created with. What it does is make sure that the vector can potentially hold numClust without having to reallocate. See here.

What you want is to either declare the vector to have that size

vector<cluster *> pclust(numClust);

or to resize the vector

pclust.resize(numClust);

Upvotes: 11

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