Most vexing parse: why doesn't A a(()); work?

Question

Among the many things Stack Overflow has taught me is what is known as the "most vexing parse", which is classically demonstrated with a line such as

A a(B()); //declares a function

While this, for most, intuitively appears to be the declaration of an object a of type A, taking a temporary B object as a constructor parameter, it's actually a declaration of a function a returning an A, taking a pointer to a function which returns B and itself takes no parameters. Similarly the line

A a(); //declares a function

also falls under the same category, since instead of an object, it declares a function. Now, in the first case, the usual workaround for this issue is to add an extra set of brackets/parenthesis around the B(), as the compiler will then interpret it as the declaration of an object

A a((B())); //declares an object

However, in the second case, doing the same leads to a compile error

A a(()); //compile error

My question is, why? Yes I'm very well aware that the correct 'workaround' is to change it to A a;, but I'm curious to know what it is that the extra () does for the compiler in the first example which then doesn't work when reapplying it in the second example. Is the A a((B())); workaround a specific exception written into the standard? Thanks.

Answer 1

There is no enlightened answer, it's just because it's not defined as valid syntax by the C++ language... So it is so, by definition of the language.

If you do have an expression within then it is valid. For example:

 ((0));//compiles

To learn more about how languages are defined, and how compilers work, you should learn about Formal language theory or more specifically Context Free Grammars (CFG) and related material like finite state machines. If you are interested in that though the wikipedia pages won't be enough, you'll have to get a book.

Answer 2

The innermost parens in your example would be an expression, and in C++ the grammar defines an expression to be an assignment-expression or another expression followed by a comma and another assignment-expression (Appendix A.4 - Grammar summary/Expressions).

The grammar further defines an assignment-expression as one of several other types of expression, none of which can be nothing (or only whitespace).

So the reason you can't have A a(()) is simply because the grammar doesn't allow it. However, I can't answer why the people who created C++ didn't allow this particular use of empty parens as some sort of special-case - I'd guess that they'd rather not put in such a special case if there was a reasonable alternative.

Answer 3

You could instead

A a(());

use

A a=A();