Funny C++ Initialization

If you have a class (call it "myclass") that has Plain Old Data (POD) members and does not have a defined constructor (i.e., uses the compiler-supplied default constructor), then "new myclass;" and "new myclass();" will do different things.  The parenthesis-free version will leave the POD uninitialized, but the with-parenthesis version will initialize the POD elements to zero.

If you define a constructor in the class, then the difference disappears.  Both forms of "new" result in leaving the POD uninitialized (as you may have originally expected).

Here's a test case that demonstrates the difference:

funny-init.cpp

Compiling (with g++) and running this little program produces:

Default constructor, no parenthesis:  42
Default constructor, parenthesis:     0
Explicit constructor, no parenthesis: 42
Explicit constructor, parenthesis:    42

As an additional bit of hilarity is that the default-constructor variant compiles to an actual object constructor that does nothing, but when you invoke "new" with parenthesis, the site of the "new" invocation is littered with extra instructions just to write zeros over the POD elements in the class.

One surprising place this difference shows up is with "struct" and placement new.  If you use placement new with a "struct" and you add the parenthesis, then the underlying storage is wiped clean.  Here's a test case for that:

placement-wipeout.cpp

And the corresponding output:

Default constructor, no parenthesis:  42
Default constructor, parenthesis:     0

It's hard to see how this is a helpful state of affairs, but forewarned is forearmed.