Dangers with unscoped multiline MACROS

Question

Is there a way to protect this MACRO from unscoped if/for statements?

#define FOO(X, Y, ...) X->bar(_##Y->thatVar, __VA_ARGS__);\
post_op(X, Y)

The order of bar and post_op is important and the result of post_op is ignorable to the application

i.e.

if(true)
    auto z = FOO(a,b);
else //compile error
    abort();

https://stackoverflow.com/questions/154136/why-use-apparently-meaningless-do-while-and-if-else-statements-in-macros — StoryTeller - Unslander Monica, May 28 '19 at 07:50
A macro spanning multiple statements cannot have a "return value". You really ought to come into better grasp with how macros behave. They are token replacement, no more. — StoryTeller - Unslander Monica, May 28 '19 at 12:03
@Yksisarvinen I over-simplified my application. Question has been edited. — Daniel, May 28 '19 at 12:03
@StoryTeller I see no real difference from token perspective; between multi-line macros and macros with return value — Daniel, May 28 '19 at 12:06
Multiple statements, not multiple lines, was my exact phrasing for a reason. Lines are immaterial. — StoryTeller - Unslander Monica, May 28 '19 at 12:07

score 3 · Accepted Answer · answered May 28 '19 at 12:10

3

Just use a lambda to reaggregate your statements back into an expression. Use it wherever an expression may appear, to your heart's content. Even in an expression statement inside an if with no braces.

#define FOO(X, Y, ...) [&] { \ 
  auto ret = X->bar(_##Y->thatVar, __VA_ARGS__);\
  post_op(X, Y); \
  return ret; }()

answered May 28 '19 at 12:10

StoryTeller - Unslander Monica

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Exactly what I was looking for. Thanx a million! – Daniel May 28 '19 at 14:22

Dangers with unscoped multiline MACROS

1 Answers1