C++ Tutorial/vector/vector reserve

Ensure there"s room for at least 1000 elements

<source lang="cpp">#include <iostream>

1. include <vector>

using namespace std; typedef vector<int> INTVECTOR; int main(void)

{
  // Dynamically allocated vector begins with 0 elements.
  INTVECTOR theVector;
  theVector.push_back(42);
  // Ensure there"s room for at least 1000 elements.
  theVector.reserve(1000);
  cout << endl << "After reserving storage for 1000 elements:" << endl;
  cout << "theVector"s size is: " << theVector.size() << endl;
  cout << "theVector"s maximum size is: " << theVector.max_size()<< endl;
  cout << "theVector"s capacity is: " << theVector.capacity() << endl;
}</source>

Reserve memory for five elements to avoid reallocation

<source lang="cpp">/* The following code example is taken from the book

* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/

1. include <iostream>
2. include <vector>
3. include <string>
4. include <algorithm>
5. include <iterator>

using namespace std; int main() {

   // create empty vector for strings
   vector<string> sentence;
   // reserve memory for five elements to avoid reallocation
   sentence.reserve(5);
   // append some elements
   sentence.push_back("Hello,");
   sentence.push_back("how");
   sentence.push_back("are");
   sentence.push_back("you");
   sentence.push_back("?");
   // print elements separated with spaces
   copy (sentence.begin(), sentence.end(),
         ostream_iterator<string>(cout," "));
   cout << endl;

}</source>

Hello, how are you ?

Reserving Space in a Vector

<source lang="cpp">#include <vector> using namespace std; int main( ) {

  // fill a vector with one million numbers the slow way
  vector<int> v1;
  for( int i = 0; i < 1000000; ++i )
     v1.push_back( i );
  // fill a vector with one million numbers the fast way
  vector<int> v2;        // has zero elements
  v2.reserve( 1000000 ); // still has zero elements
  for( int i = 0; i < 1000000; ++i )
     v2.push_back( i );

}</source>