C/C++ Syntax - Separating statements with , instead of ; legal?

Question

I just ran into this piece of code that does this :

delete a, a = 0;

It compiles and runs just fine. But isn't this supposed to be :

delete a;
a = 0;

Why is separating statements using , allowed in this case ?

Thanks :)

Answer 1

In C and C++, most "statements" are actually expressions. The semicolon added to an expression makes it into a statement. Alternatively, it is allowed (but almost always bad style) to separate side-effectful expressions with the comma operator: the left-hand-side expression is evaluated for its side-effects (and its value is discarded), and the right-hand-side expression is evaluated for its value.

Answer 2

This is the comma-operator. It evaluates both it's arguments and returns the second one.

Answer 3

This is the comma operator. It can be used to separate expressions, but not declarations.

Answer 4

That is comma operator. MSDN article is here. And have a look at this question to understand how it works.

Answer 5

While it is possible to write code like that, it may be somewhat weird. A slightly more realistic usecase would be if you have a struct T as follows:

struct T {
    bool check() const;
    void fix();
};

Now you want to iterate through everything in the struct and run check on it, and then call fix if check returns false. The simple way to do this would be

for (list<T>::iterator it = mylist.begin(); it < mylist.end(); ++it)
     if (!it->check())
         it->fix();

Let's pretend you want to write it in as short a way as possible. fix() returning void means you can't just put it in the condition. However, using the comma operator you can get around this:

for (auto it = mylist.begin(); it != mylist.end() && (it->check() || (it->fix(), true)); ++it);

I wouldn't use it without a particularly good reason, but it does allow you to call any function from a condition, which can be convenient.