CODEWITHSUNDEEP
c++vector
bcf
bcf

Reputation: 2134

Most efficient way to overwrite a vector with a larger one

I have a std::vector I want to overwrite with new elements that will make it larger as well. What would be the fastest/most memory efficient method to do this? Here's my naive implementation:

#include <vector>

int main() {
  std::vector<int> smallVec(10, 1);

  int bigVecSize = 100;

  smallVec.clear();         
  for(int i = 0; i < bigVecSize; ++i)
    smallVec.push_back(i);
}

Are there any C++11 features that might help? Thanks.

Upvotes: 2

Views: 812

Answers (2)

Alex Chi
Alex Chi

Reputation: 736

Sorry, I don't have any idea with C++11.

But, I think it is more faster to copy memory by ::memcpy.

#include <vector>

void overwrite(std::vector<int>& dst, const std::vector<int>& src)
{
    /* some code - to check the valid of dst and src */

    dst.resize(src.size());
    ::memcpy(dst.data(), src.data(), sizeof(int) * src.size());
}

int main(int argc, char* argv[])
{
    // an example
    std::vector<int> via, vib;
    via.push_back(22);
    via.resize(5, 1);
    vib.push_back(123);
    vib.resize(10, 2);
    vib.push_back(123);
    overwrite(via, vib);
    return 0;
}

The data memory of std::vector is always consequent, so you can copy memory from one std::vector to another.

Note: You need to be careful when the item type of your std::vector is the object what contain a/some pointer(s). Because the ::memcpy just copy the address to another, don't copy the object of pointer.

Upvotes: 3

michaeltang
michaeltang

Reputation: 2898

smallVec.reserve(bigVecSize); can reserve enough space for the new elements.

try this code :

#include <vector>
#include <stdio.h> //for printf()
#include <stdlib.h> //for system()
#include <time.h> //for time() & time_t
int main() {
  std::vector<int> smallVec(10, 1);
  int bigVecSize = 1000000;
  smallVec.reserve(bigVecSize);
  smallVec.clear();

  time_t ts,te;
  ts=time(NULL);

  for(int i = 0; i < bigVecSize; ++i)
    smallVec.push_back(i);

  te=time(NULL);
  printf("%ld\n",te-ts);
}

use reserve reduce the time cost from 0.125s to 0.087s

in C++11 if user define element is used as element of vector , R-value reference can be used to reduce copy.

Upvotes: 1

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