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Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?
I've been seeing that expression for over 10 years now. I've been trying to think what it's good for. Since I see it mostly in #defines, I assume it's good for inner scope variable declaration and for using breaks (instead of gotos.)
Is it good for anything else? Do you use it?
It's the only construct in C that you can use to #define a multistatement operation, put a semicolon after, and still use within an if statement. An example might help:
#define FOO(x) foo(x); bar(x)
if (condition)
FOO(x);
else // syntax error here
...;
Even using braces doesn't help:
#define FOO(x) { foo(x); bar(x); }
Using this in an if statement would require that you omit the semicolon, which is counterintuitive:
if (condition)
FOO(x)
else
...
If you define FOO like this:
#define FOO(x) do { foo(x); bar(x); } while (0)
then the following is syntactically correct:
if (condition)
FOO(x);
else
....
It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function. Suppose you have:
#define FOO(n) foo(n);bar(n)
and you do:
void foobar(int n) {
if (n)
FOO(n);
}
then this expands to:
void foobar(int n) {
if (n)
foo(n);bar(n);
}
Notice that the second call bar(n) is not part of the if statement anymore.
Wrap both into do { } while(0), and you can also use the macro in an if statement.
Generically, do/while is good for any sort of loop construct where one must execute the loop at least once. It is possible to emulate this sort of looping through either a straight while or even a for loop, but often the result is a little less elegant. I'll admit that specific applications of this pattern are fairly rare, but they do exist. One which springs to mind is a menu-based console application:
do {
char c = read_input();
process_input(c);
} while (c != 'Q');
It is a way to simplify error checking and avoid deep nested if's. For example:
do {
// do something
if (error) {
break;
}
// do something else
if (error) {
break;
}
// etc..
} while (0);
It is interesting to note the following situation where the do {} while (0) loop won't work for you:
If you want a function-like macro that returns a value, then you will need a statement expression: ({stmt; stmt;}) instead of do {} while(0):
#include <stdio.h>
#define log_to_string1(str, fmt, arg...) \
do { \
sprintf(str, "%s: " fmt, "myprog", ##arg); \
} while (0)
#define log_to_string2(str, fmt, arg...) \
({ \
sprintf(str, "%s: " fmt, "myprog", ##arg); \
})
int main() {
char buf[1000];
int n = 0;
log_to_string1(buf, "%s\n", "No assignment, OK");
n += log_to_string1(buf + n, "%s\n", "NOT OK: gcc: error: expected expression before 'do'");
n += log_to_string2(buf + n, "%s\n", "This fixes it");
n += log_to_string2(buf + n, "%s\n", "Assignment worked!");
printf("%s", buf);
return 0;
}
Generically, do/while is good for any sort of loop construct where one must execute the loop at least once. It is possible to emulate this sort of looping through either a straight while or even a for loop, but often the result is a little less elegant. I'll admit that specific applications of this pattern are fairly rare, but they do exist. One which springs to mind is a menu-based console application:
do {
char c = read_input();
process_input(c);
} while (c != 'Q');
It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function. Suppose you have:
#define FOO(n) foo(n);bar(n)
and you do:
void foobar(int n) {
if (n)
FOO(n);
}
then this expands to:
void foobar(int n) {
if (n)
foo(n);bar(n);
}
Notice that the second call bar(n) is not part of the if statement anymore.
Wrap both into do { } while(0), and you can also use the macro in an if statement.
It is interesting to note the following situation where the do {} while (0) loop won't work for you:
If you want a function-like macro that returns a value, then you will need a statement expression: ({stmt; stmt;}) instead of do {} while(0):
#include <stdio.h>
#define log_to_string1(str, fmt, arg...) \
do { \
sprintf(str, "%s: " fmt, "myprog", ##arg); \
} while (0)
#define log_to_string2(str, fmt, arg...) \
({ \
sprintf(str, "%s: " fmt, "myprog", ##arg); \
})
int main() {
char buf[1000];
int n = 0;
log_to_string1(buf, "%s\n", "No assignment, OK");
n += log_to_string1(buf + n, "%s\n", "NOT OK: gcc: error: expected expression before 'do'");
n += log_to_string2(buf + n, "%s\n", "This fixes it");
n += log_to_string2(buf + n, "%s\n", "Assignment worked!");
printf("%s", buf);
return 0;
}
It is a way to simplify error checking and avoid deep nested if's. For example:
do {
// do something
if (error) {
break;
}
// do something else
if (error) {
break;
}
// etc..
} while (0);
Generically, do/while is good for any sort of loop construct where one must execute the loop at least once. It is possible to emulate this sort of looping through either a straight while or even a for loop, but often the result is a little less elegant. I'll admit that specific applications of this pattern are fairly rare, but they do exist. One which springs to mind is a menu-based console application:
do {
char c = read_input();
process_input(c);
} while (c != 'Q');
It is a way to simplify error checking and avoid deep nested if's. For example:
do {
// do something
if (error) {
break;
}
// do something else
if (error) {
break;
}
// etc..
} while (0);
Source: Stackoverflow.com