Editing Generator (computer programming) (section)

===C++===

It is possible to introduce generators into C++ using pre-processor macros. The resulting code might have aspects that are very different from native C++, but the generator syntax can be very uncluttered.<ref>{{Cite web|url=http://www.codeproject.com/KB/cpp/cpp_generators.aspx|title = Generators in C++|date = 21 September 2008}}</ref> The set of pre-processor macros defined in this source allow generators defined with the syntax as in the following example:

<syntaxhighlight lang="cpp">
$generator(descent)
{
   int i;

   // place the constructor of our generator, e.g. 
   // descent(int minv, int maxv) {...}
   
   // from $emit to $stop is a body of our generator:
    
   $emit(int) // will emit int values. Start of body of the generator.
      for (i = 10; i > 0; --i)
         $yield(i); // similar to yield in Python,
                    // returns next number in [1..10], reversed.
   $stop; // stop, end of sequence. End of body of the generator.
};
</syntaxhighlight>

This can then be iterated using:

<syntaxhighlight lang="cpp">
int main(int argc, char* argv[])
{
  descent gen;
  for (int n; gen(n);) // "get next" generator invocation
    printf("next number is %d\n", n);
  return 0;
}
</syntaxhighlight>

Moreover, [[C++11]] allows [[foreach loop]]s to be applied to any class that provides the <code>begin</code> and <code>end</code> functions. It's then possible to write generator-like classes by defining both the iterable methods (<code>begin</code> and <code>end</code>) and the iterator methods (<code>operator!=</code>, <code>operator++</code> and <code>operator*</code>) in the same class. For example, it is possible to write the following program:

<syntaxhighlight lang="cpp">
#include <iostream>
int main()
{
    for (int i: range(10))
    {
        std::cout << i << std::endl;
    }
    return 0;
}
</syntaxhighlight>

A basic range implementation would look like that:

<syntaxhighlight lang="cpp">
class range
{
private:
    int last;
    int iter;

public:
    range(int end):
        last(end),
        iter(0)
    {}

    // Iterable functions
    const range& begin() const { return *this; }
    const range& end() const { return *this; }

    // Iterator functions
    bool operator!=(const range&) const { return iter < last; }
    void operator++() { ++iter; }
    int operator*() const { return iter; }
};
</syntaxhighlight>