What is the purpose of the Most Vexing Parse?

Solution 1:

Let's say MVP didn't exist.

How would you declare a function?

A foo();

would be a variable definition, not a method declaration. Would you introduce a new keyword? Would you have a more awkward syntax for a function declaration? Or would you rather have

A foo;

define a variable and

A foo();

declare a function?

Your slightly more complicated example is just for consistency with this basic one. It's easier to say "everything that can be interpreted as a declaration, will be interpreted as a declaration" rather than "everything that can be interpreted as a declaration, will be interpreted as a declaration, unless it's a single variable definition, in which case it's a variable definition".

This probably isn't the motivation behind it though, but a reason it's a good thing.

Solution 2:

For C++, it's pretty simple: because the rule was made that way in C.

In C, the ambiguity only arises with a typedef and some fairly obscure code. Almost nobody ever triggers it by accident -- in fact, it probably qualifies as rare except in code designed specifically to demonstrate the possibility. For better or worse, however, the mere possibility of the ambiguity meant somebody had to resolve it -- and if memory serves, it was resolved by none other than Dennis Ritchie, who decreed that anything that could be interpreted as a declaration would be a declaration, even if there was also an ambiguous interpretation as a definition.

C++ added the ability to use parentheses for initialization as well as function calls as grouping, and this moved the ambiguity from obscure to common. Changing it, however, would have required breaking the rule as it came from C. Resolving this particular ambiguity as most would expect, without creating half a dozen more that were even more surprising would probably have been fairly non-trivial as well, unless you were willing to throw away compatibility with C entirely.