How to declare'n'run lambda in a single expression in C++11?

Question

Like almost all self-respecting projects, my own one also have macro to check some conditions in Debug mode:

#ifndef NDEBUG
#  define DCHECK(x) if (!(x)) { ... }
#else
#  define DCHECK(x)
#endif

But now I want to DCHECK() some complex conditions, like file permissions:

...
auto has_permissions = [fd] {
  struct stat st;
  if (fstat(fd, &st) == 0) {
    return (st.st_mode & (S_IRUSR | S_IWUSR)) == (S_IRUSR | S_IWUSR);
  }
  return false;
};
DCHECK(has_permissions());
...

Also, I don't want to pollute my code with lambdas which are not used anywhere besides a sole DCHECK() , since in Release mode they will be marked as unused variables.

So the question is: is there a compact way to write DCHECK() statement that declares, runs and checks any complex condition?

PS It's not a mandatory to use lambdas - it's just my proposal.

PPS DCHECK() may be reimplemented, but all already existing invocations should still work without modifications.

Answer 1

Define the lambda within the DCHECK macro and invoke it

DCHECK(([]{ return false; }()));
//                         ^^

Live demo