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CppForEach

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(C++) std::for_each

Algorithm to perform a function on a sequence of values (e.g. a vector. Prefer algorithms over hand-written loops [1,2].

There are two kinds of examples below. The first uses std::for_each combined with simple functions.

The second piece of code shows the use of functors for more advanced functionality. It is advised to use the latter [3], but I will show the first as an example. There are also STL functors.

Example: Use of plain functions

include <iostream>

include <vector>

include <algorith>


void setToOne(int& i)
{
  i = 1;
}
void multiplyByTwo(int& i)
{
  i*=2;
}
void setToRandom(int& i)
{
  i = std::rand()%10;
}
void coutVector(const std::vector<int>& myVector)
{
  const int size = myVector.size();
  for (int i=0; i!=size; ++i)
  {
    std::cout << i << " : " << myVector[i] << std::endl;
  }
}
int main()
{
  const int size = 5;
  std::vector<int> myVector(size);
  std::for_each(myVector.begin(),myVector.end(), setToOne);
  coutVector(myVector);
  std::for_each(myVector.begin(),myVector.end(), multiplyByTwo);
  coutVector(myVector);
  std::for_each(myVector.begin(),myVector.end(), setToRandom);
  coutVector(myVector);
  std::for_each(myVector.begin(),myVector.end(), multiplyByTwo);
  coutVector(myVector);
  return 0;
}

Example: Use of functors

The preffered way [3].

include <iostream>

include <vector>

include <algorith>


class MyInitializer
{
  public:
  MyInitializer()
  : index(0) {}
  template<class T>
  void operator () (T & a)
  {
    a = index;
    ++index;
  }
  private:
  int index;
};
class MyIndexCout
{
  public:
  MyIndexCout() : index(0) {}
  template<class T>
  void operator () (const T & a)
  {
    std::cout << index << " : " << a << std::endl;
    ++index;
  }
  private:
  int index;
};
class MySquarer
{
  public:
  MySquarer() {}
  template<class T> void operator () (T & a)
  {
    a*=a;
  }
};
int main()
{
  const int size = 10;
  std::vector<int> myVector(size);
  std::for_each(myVector.begin(), myVector.end(), MyInitializer());
  std::for_each(myVector.begin(), myVector.end(), MyIndexCout());
  std::cout << "-----------" << std::endl;
  std::for_each(myVector.begin(), myVector.end(), MySquarer());
  std::for_each(myVector.begin(), myVector.end(), MyIndexCout());
}

References

• 1) Bjarne Stroustrup. The C++ Programming Language (3rd edition).ISBN: 0-201-88954-4 18.12.1 : 'Prefer algorithms over loops'.
• 2) Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 84: 'Prefer algorithm calls to handwritten loops.'
• 3) Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 88: 'Prefer function objects over functions as algorithm and comparer arguments.'

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