I have read in some other questions here in the forum why do { } while (0) is good for function-like macros in C. Also, this form provides the break instruction in order to simulate a return statement.
But in those other questions, I never got anything "clear" about if (1) { } else.
I mean, for me, this is almost equal to do { } while (0) with the difference that here we don't have the break statement.
And I guess it’s better than if (1) { } else { } because allows us to use ;.
But I want to ask you, is it really "safe"? Are there any reasons for choosing this over do { } while (0)?
Consider the alternatives you propose:
#define foo(x) do { /* main part of macro here */ } while (0)
#define foo(x) if (1) { /* main part of macro here */ } else
Both of these serve the purpose that the macro can be written like a function call:
foo(x);
Notably, the semicolon ends the statement, so the macro invocation looks like many other C statements. To somebody experienced in C, it looks odd if there is a line without a semicolon, like:
x = 3;
foo(x)
y = 4;
However, consider if somebody does accidentally write the above, perhaps because they knew foo was a macro and thought it was a whole statement by itself. Then the while form becomes:
x = 3;
do { /* main part of macro here */ } while (0)
y = 4;
This generates a compiler error since it is not valid C grammar (barring weird extensions). In contrast, the if form becomes:
x = 3;
if (1) { /* main part of macro here */ } else
y = 4;
This may compile without an error, since the y = 4; becomes part of the else. So, in this respect, the while form is superior since it catches an error that the if form does not.