c ++向量不像数组

Question

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?

Answer 1

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);

Answer 2

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? :)

Answer 3

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.

Answer 4

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);