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

I've used it in a compile-time assertion macro to have the macro create a name for a typedef that will be unique. See

• Ways to ASSERT expressions at build time in C

if you want the gory details.

__COUNTER__ is useful anywhere you need a unique name. I have used it extensively for RAII style locks and stacks. Consider:

struct TLock
{
  void Lock();
  void Unlock();
}
g_Lock1, g_Lock2;

struct TLockUse
{
  TLockUse( TLock &lock ):m_Lock(lock){ m_Lock.Lock(); }
  ~TLockUse(){ m_Lock.Unlock(); }

  TLock &m_Lock;
};

void DoSomething()
{
  TLockUse lock_use1( g_Lock1 );
  TLockUse lock_use2( g_Lock2 );
  // ...
}

It gets tedious to name the lock uses, and can even become a source of errors if they're not all declared at the top of a block. How do you know if you're on lock_use4 or lock_use11? It's also needless pollution of the namespace - I never need to refer to the lock use objects by name. So I use __COUNTER__:

#define CONCAT_IMPL( x, y ) x##y
#define MACRO_CONCAT( x, y ) CONCAT_IMPL( x, y )
#define USE_LOCK( lock ) TLockUse MACRO_CONCAT( LockUse, __COUNTER__ )( lock )

void DoSomething2()
{
  USE_LOCK( g_Lock1 );
  USE_LOCK( g_Lock2 );
  // ...
}

But don't get hung up on the fact I called the objects locks - any function(s) that need to get called in matching pairs fit this pattern. You might even have multiple uses on the same "lock" in a given block.

I've never used it for anything but a DEBUG macro. It's convenient to be able to say

#define WAYPOINT \
    do { if(dbg) printf("At marker: %d\n", __COUNTER__); } while(0);