What are the most surprising elements of the C++ standard?

Question

I've decided to get more acquainted with my favorite programming language, but only reading the standard is boring.

What are the most surprising, counter-intuitive, or just plain weird elements of C++? What has shocked you enough that you ran to your nearest compiler to check if it's really true?

I'll accept the first answer that I won't believe even after I've tested it. :)

Answer 1

I found it somewhat surprising that

class aclass
{
public:
int a;
};

some_function(aclass());

will have ainitialized to 0 in some_function, while

aclass ac;
some_function(ac);

will leave it unitinitalized. If you explicitly define the default constructor of aclass:

class aclass
{
public:
aclass(): a() {}
int a;
};

then aclass ac; will also initialize a to 0.

Answer 2

Another answer I could add would be the throw() qualifier. For example:

void dosomething() throw()
{
   // ....
}

void doSomethingElse() throw(std::exception)
{
  // ....
}

Intuitively, this seems like a contract with the compiler, indicating that this function is not allowed to throw any exceptions except for those listed. But in reality, this does nothing at compile time. Rather it is a run-time mechanism and it will not prevent the function from actually throwing an exception. Even worse, if an unlisted exception is thrown, it terminates your application with a call to std::terminate().