Uses of C comma operator [duplicate]

C language (as well as C++) is historically a mix of two completely different programming styles, which one can refer to as "statement programming" and "expression programming". As you know, every procedural programming language normally supports such fundamental constructs as sequencing and branching (see Structured Programming). These fundamental constructs are present in C/C++ languages in two forms: one for statement programming, another for expression programming.

For example, when you write your program in terms of statements, you might use a sequence of statements separated by ;. When you want to do some branching, you use if statements. You can also use cycles and other kinds of control transfer statements.

In expression programming the same constructs are available to you as well. This is actually where , operator comes into play. Operator , in nothing else than a separator of sequential expressions in C, i.e. operator , in expression programming serves the same role as ; does in statement programming. Branching in expression programming is done through ?: operator and, alternatively, through short-circuit evaluation properties of && and || operators. (Expression programming has no cycles though. And to replace them with recursion you'd have to apply statement programming.)

For example, the following code

a = rand();
++a;
b = rand();
c = a + b / 2;
if (a < c - 5)
  d = a;
else
  d = b;

which is an example of traditional statement programming, can be re-written in terms of expression programming as

a = rand(), ++a, b = rand(), c = a + b / 2, a < c - 5 ? d = a : d = b;

or as

a = rand(), ++a, b = rand(), c = a + b / 2, d = a < c - 5 ? a : b;

or

d = (a = rand(), ++a, b = rand(), c = a + b / 2, a < c - 5 ? a : b);

or

a = rand(), ++a, b = rand(), c = a + b / 2, (a < c - 5 && (d = a, 1)) || (d = b);

Needless to say, in practice statement programming usually produces much more readable C/C++ code, so we normally use expression programming in very well measured and restricted amounts. But in many cases it comes handy. And the line between what is acceptable and what is not is to a large degree a matter of personal preference and the ability to recognize and read established idioms.

As an additional note: the very design of the language is obviously tailored towards statements. Statements can freely invoke expressions, but expressions can't invoke statements (aside from calling pre-defined functions). This situation is changed in a rather interesting way in GCC compiler, which supports so called "statement expressions" as an extension (symmetrical to "expression statements" in standard C). "Statement expressions" allow user to directly insert statement-based code into expressions, just like they can insert expression-based code into statements in standard C.

As another additional note: in C++ language functor-based programming plays an important role, which can be seen as another form of "expression programming". According to the current trends in C++ design, it might be considered preferable over traditional statement programming in many situations.

I think generally C's comma is not a good style to use simply because it's so very very easy to miss - either by someone else trying to read/understand/fix your code, or you yourself a month down the line. Outside of variable declarations and for loops, of course, where it is idiomatic.

You can use it, for example, to pack multiple statements into a ternary operator (?:), ala:

int x = some_bool ? printf("WTF"), 5 : fprintf(stderr, "No, really, WTF"), 117;

but my gods, why?!? (I've seen it used in this way in real code, but don't have access to it to show unfortunately)

I've seen it used in macros where the macro is pretending to be a function and wants to return a value but needs to do some other work first. It's always ugly and often looks like a dangerous hack though.

Simplified example:

#define SomeMacro(A) ( DoWork(A), Permute(A) )

Here B=SomeMacro(A) "returns" the result of Permute(A) and assigns it to "B".

Two killer comma operator features in C++:

a) Read from stream until specific string is encountered (helps to keep the code DRY):

 while (cin >> str, str != "STOP") {
   //process str
 }

b) Write complex code in constructor initializers:

class X : public A {
  X() : A( (global_function(), global_result) ) {};
};

The Boost Assignment library is a good example of overloading the comma operator in a useful, readable way. For example:

using namespace boost::assign;

vector<int> v; 
v += 1,2,3,4,5,6,7,8,9;