Real-world use of X-Macros

Question

I just learned of X-Macros. What real-world uses of X-Macros have you seen? When are they the right tool for the job?

Answer 1

Some real-world uses of X-Macros by popular and large projects:

Java HotSpot

In the Oracle HotSpot Virtual Machine for the Java® Programming Language, there is the file globals.hpp, which uses the RUNTIME_FLAGS in that way.

See the source code:

• JDK 7
• JDK 8
• JDK 9

Chromium

The list of network errors in net_error_list.h is a long, long list of macro expansions of this form:

NET_ERROR(IO_PENDING, -1)

It is used by net_errors.h from the same directory:

enum Error {
  OK = 0,

#define NET_ERROR(label, value) ERR_ ## label = value,
#include "net/base/net_error_list.h"
#undef NET_ERROR
};

The result of this preprocessor magic is:

enum Error {
  OK = 0,
  ERR_IO_PENDING = -1,
};

What I don't like about this particular use is that the name of the constant is created dynamically by adding the ERR_. In this example, NET_ERROR(IO_PENDING, -100) defines the constant ERR_IO_PENDING.

Using a simple text search for ERR_IO_PENDING, it is not possible to see where this constant it defined. Instead, to find the definition, one has to search for IO_PENDING. This makes the code hard to navigate and therefore adds to the obfuscation of the whole code base.