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Creating your own STL-style algorithms

Once you become comfortable with the STL algorithm style, you can begin to create your own STL-style algorithms. Because these will conform to the format of all the other algorithms in the STL, they’re easy to use for programmers who are familiar with the STL, and thus become a way to “extend the STL vocabulary.”

The easiest way to approach the problem is to go to the <algorithm> header file and find something similar to what you need, and modify that (virtually all STL implementations provide the code for the templates directly in the header files). For example, an algorithm that stands out by its absence is copy_if( ) (the closest approximation is partition( )), which was used in Binder1.cpp at the beginning of this chapter, and in several other examples in this chapter. This will only copy an element if it satisfies a predicate. Here’s an implementation: 

//: C21:copy_if.h
// Roll your own STL-style algorithm
#ifndef COPY_IF_H
#define COPY_IF_H

template<typename ForwardIter,
  typename OutputIter, typename UnaryPred>
OutputIter copy_if(ForwardIter begin, ForwardIter end,
  OutputIter dest, UnaryPred f) {
  while(begin != end) {
    if(f(*begin))
      *dest++ = *begin;
    begin++;
  }
  return dest;
}
#endif // COPY_IF_H ///:~

The return value is the past-the-end iterator for the destination sequence (the copied sequence).

Now that you’re comfortable with the ideas of the various iterator types, the actual implementation is quite straightforward. You can imagine creating an entire additional library of your own useful algorithms that follow the format of the STL.

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