CODEWITHSUNDEEP
c++
q0987
q0987

Reputation: 35984

C++ -- Get internal pointer to a vector

I use the following method to get an allocated memory space without worrying about how to reclaim the allocated resource.

#include <vector>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    vector<int> vecInts;

    for(int iInd=0; iInd<10; i++)
        vecInts.push_back(iInd);

    int* pInt = &vecInts[0]; // Is this a good method?

    // now can I use pInt to modify the value of the vecInts?
    // I will NOT resize the vector and just simply manipulate the values inside
    return 0;
}

However, I am not sure whether such a method is a good one or not.

Thank you

Upvotes: 5

Views: 4200

Answers (2)

DS.
DS.

Reputation: 24110

That's a fine method. The memory will be freed automatically when the vector goes out of scope. In modern STL implementations vector has a method data() which returns the pointer to the start of the memory, which is equivalent to &v[0], but looks much cleaner.

This approach also has the nice property that each element is set to 0 (or default-constructed if it's an object).

For efficiency, if you know the length of the vector in advance, you can supply it as a constructor argument. Then all the memory will be allocated once. E.g.

vector<char> buffer(bufSize);
fread(buffer.data(), 1, buffer.size(), file);

And, like you mention in the comment, be aware that buffer.data() will change if the buffer is resized, and deallocated when it goes out of scope.

Upvotes: 3

Tony Delroy
Tony Delroy

Reputation: 106068

Yes, that's fine given the precautions you mention (and other obvious ones, like not using a pointer into a vector that's gone out of scope etc.). The pointer will be valid and invalidated in the exact same way that an iterator into the container would be.

Upvotes: 5

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