The C++ Lambda Preprocessor

Tired of waiting around for anonymous closures to arrive in C++0x (or at least in GCC)? Boost.Lambda and other hacks leaving you pining for something with free variable capture? Get them today with the amazing C++ Lambda Preprocessor (“clamp”)!

Here’s a little demo illustrating the use of lambdas with threads—something which may be more interesting to systems programmers than the simple accumulator examples on the clamp homepage.

#include <boost/thread.hpp>
#include <iostream>

using namespace boost;
using namespace std;
#include "lambda_impl.clamp_h"

enum { n = 3 };
int main() {
  const string msgs[n] = {"first", "second", "third"};
  mutex m;
  for (int i = 0; i < n; ++i) {
    boost::thread(lambda() {
      mutex::scoped_lock l(__ref(m));
      cout << "message from thread " << __ctx(i)
           << ": " << __ref(msgs[i]) << endl;
  return 0;

// clamp < | sed '1d' >
// g++  -lboost_thread-gcc43-mt -o clamp2
// ./clamp2

There are a few annoyances with clamp:

  • All free variables must be explicitly captured with __ref() or __ctx(). This is because clamp doesn’t try to actually parse your C++ (understandably). (Not parsing the C++ also makes it more flexible, accommodating various language extensions.) Contrast this with C++0x, where you can use short-hand notations like [&], which captures all free variables by reference (perhaps the most common case).
  • You will most likely need to muck with the output. clamp generates a number of header files that form the first #include in your source file. However, if you ever refer to global types/variables/functions, you will need to reorder things, add forward declarations, separate the generated header contents into declarations and definitions, etc. This is made nearly trivial, however, if you also use Lazy C++, which I’ll cover next time.

Despite these minor quibbles, clamp is simple and effective. The generated code is straightforward—pretty much exactly what you would expect to write yourself by hand.

Follow me on Twitter for stuff far more interesting than what I blog.