Wijaa asks on stackoverflow: What’s the most brilliant use of templates you’ve ever encountered?

Reading the linked articles gave me much joy. Here are some highlights:

• In FOREACH Redux, Eric Niebler describes his long path to the BOOST_FOREACH macro, which emulates C#’s foreach statement in C++. At the core, it all revolves around using the conditional operator ?: with templates to work the compilers type inference engine:

  
  struct rvalue_probe
  {
     template< class R > operator R ()
     {
          throw "rvalue";
     }
     template< class L > operator L &amp; () const
     {
          throw "lvalue";
     }
  };
  
  #define RVALUE_TEST(container) \
     try { ( true ? rvalue_probe() : (container) ); } \
     catch( char const * result ) { cout <<result << '\n'; }
  

  This snippet for example uses the overload resolution rules to decide whether container is an rvalue or an lvalue, without evaluating it. For a detailed explanation, read the full article.

• The next link leads to the Spirit parser generator’s quick start. Except that Spirit is no parser generator like yacc. All rules are directly defined in C++. This one-liner calls f every time a real number is recognized. Multiple numbers are separated with comma.
  
real_p[&f] >> *(',' >> real_p[&f])

real_p is a parser for real numbers, >> puts parsers into sequences and * is the Kleene Star. Read the quick start for an explanation how this all fits together.

So whatever you do, don’t say C++ didn’t make any progress.

Have fun and a nice Sunday!

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