Has anyone ever had a use for the __COUNTER__ pre-processor macro?

Question

The __COUNTER__ symbol is provided by VC++ and GCC, and gives an increasing non-negative integral value each time it is used.

I\'m interested to learn w

Answer 1

I intend to use __COUNTER__ to give every file in our codebase a unique identifier, so that that unique code can be used in logging ASSERTs in an embedded system.

This method is much more efficient than using strings to store filenames (using __FILE__), especially on an embedded system with tiny ROM. I thought about the idea whilst I was reading this article - Assert Yourself on Embedded.com. It's a shame that it only works with GCC-based compilers though.

Answer 2

It is used by Boost.Asio to implement stackless coroutines.

See this header file and examples.

Resulting coroutines look like this:

struct task : coroutine
{
  ...
  void operator()()
  {
    reenter (this)
    {
      while (... not finished ...)
      {
         ... do something ...
         yield;
         ... do some more ...
         yield;
       }
     }
   }
   ...
};

Answer 3

In this blog post it is used for simulating the defer statement of golang in C++11.

template <typename F>
struct privDefer {
    F f;
    privDefer(F f) : f(f) {}
    ~privDefer() { f(); }
};

template <typename F>
privDefer<F> defer_func(F f) {
    return privDefer<F>(f);
}

#define DEFER_1(x, y) x##y
#define DEFER_2(x, y) DEFER_1(x, y)
#define DEFER_3(x)    DEFER_2(x, __COUNTER__)
#define defer(code)   auto DEFER_3(_defer_) = defer_func([&](){code;})

Then you can do:

int main()
{
    FILE* file = open("file.txt");
    defer(fclose(file));

    // use the file here
    // ....
}

Answer 4

I've used it to generate unique types in this article: http://www.codeproject.com/Articles/42021/Sealing-Classes-in-C

Answer 5

If I'm understanding the functionality correctly, I wished I had that functionality when I was working in Perl, adding an Event Logging function into an existing GUI. I wanted to ensure that the needed hand testing (sigh) gave us complete coverage, so I logged every test point to a file, and logging a __counter__ value made it easy to see what was missing in the coverage. As it was, I hand coded the equivalent.

Answer 6

A usage is in TensorFlow's REGISTER_KERNEL_BUILDER macro. Each TensorFlow Op could have one or more kernels as its implementations. These kernels are registered with a registrar. The registration of a kernel is done by defining a global variable -- the constructor of the variable can do the registration. Here the authors use __COUNTER__ to give each global variable a unique name.

#define REGISTER_KERNEL_BUILDER(kernel_builder, ...) \
  REGISTER_KERNEL_BUILDER_UNIQ_HELPER(__COUNTER__, kernel_builder, __VA_ARGS__)

#define REGISTER_KERNEL_BUILDER_UNIQ_HELPER(ctr, kernel_builder, ...) \
  REGISTER_KERNEL_BUILDER_UNIQ(ctr, kernel_builder, __VA_ARGS__)

#define REGISTER_KERNEL_BUILDER_UNIQ(ctr, kernel_builder, ...)          \
  static ::tensorflow::kernel_factory::OpKernelRegistrar                \
  registrar__body__##ctr##__object(                                 \
      SHOULD_REGISTER_OP_KERNEL(#__VA_ARGS__)                       \
      ? ::tensorflow::register_kernel::kernel_builder.Build()   \
      : nullptr,                                                \
      #__VA_ARGS__, [](::tensorflow::OpKernelConstruction* context) \
            -> ::tensorflow::OpKernel* {                \
              return new __VA_ARGS__(context);          \
            });