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c++memorystdvector

How does vectors in C++ use memory?

#include <iostream>
#include <vector>

int main(int argc, char const *argv[])
{
    std::vector<int> a(32768, 0);
    std::cout << "Size " << sizeof a << "\nCapacity " << a.capacity() << "\nElements " << a.size();
    return 0;
}

for this program im getting the output:

Size 24
Capacity 32768
Elements 32768

using valgrind i calculated heap usage which is:

132096 bytes

that is (32768 x 4 bytes) + 24 bytes

im interested in how are these 24 bytes used by vector a

Solution

  • As addressed in the comments by Kamil, a std::vector keeps track of three pointers internally. One pointer to the begin, one to end and one to the end of allocated memory (see stack post). Now, the size of a pointer should be 8 bytes on any 64-bit C/C++ compiler so, 3 * 8 bytes = 24 bytes (see wiki).