cpp::daily - std::set, std::multiset e std::move

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cpp::daily - std::set, std::multiset e std::move

std::set

It is a kind of associative container where each element must be unique. The element’s value cannot be modified once it has been added to the set, although it is possible to remove and add the modified value from that element.

Example:

#include <iostream>
#include <set>

int main(){
  std::set<int> s;

  // adding elements to the set
  s.insert( 11 );
  s.insert( 13 );
  s.insert( 17 );
  s.insert( 8 );

  // Prints all elements of the set
  // Will print from largest to smallest by default
  for( auto z : s ){
    std::cout << z << ' ';
  }
  std::cout << '\n';

  // remove elements from the set 
  s.erase( 8 );
  s.erase( 13 );

  // Prints all elements of the set after removing some
  for( auto z : s ){
    std::cout << z << ' ';
  }

  std::cout << '\n';
  return 0;
}

std::multiset

It is similar to std::set, but stores duplicate elements, eg:

With std::set if we double the numbers like this:

#include <iostream>
#include <set>

int main(){
  std::set<int> s;

  s.insert( 11 );
  s.insert( 13 );
  s.insert( 17 );
  s.insert( 8 );
  s.insert( 11 );
  s.insert( 13 );
  s.insert( 17 );
  s.insert( 8 );


  for( auto z : s ){
    std::cout << z << ' ';
  }
  std::cout << '\n';

  return 0;
}

The output will be: 8 11 13 17 , that is, it will not be duplicated.

But if we use std::multiset, like this:

#include <iostream>
#include <set>

int main(){
  std::multiset<int> m;

  m.insert( 11 );
  m.insert( 13 );
  m.insert( 17 );
  m.insert( 8 );
  m.insert( 11 );
  m.insert( 13 );
  m.insert( 17 );
  m.insert( 8 );


  for( auto z : m ){
    std::cout << z << ' ';
  }
  std::cout << '\n';

  return 0;
}

std::move

To understand std::move let’s remember that cpp::daily we swapped variable values with std::swap .

Actually that was possible using a temporary variable, so it had a memory copy of the value. Already with std::move the exchange is really done without copying, example based on that:

T tmp(std::move(a));
a = std::move(b);   
b = std::move(tmp);

This performs better than that example.

That’s all for today, small daily doses that will always keep us in tune with C++!

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