C++ new

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struct A { int m; }; // POD
struct B { ~B(); int m; }; // non-POD, compiler generated default ctor
struct C { C() : m() {}; ~C(); int m; }; // non-POD, default-initialising m

In a C++03 conformant compiler, things should work like so:

• new A - indeterminate value
• new A() - value-initialize A, which is zero-initialization since it’s a POD.
• new B - default-initializes (leaves B::m uninitialized)
• new B() - value-initializes B which zero-initializes all fields since its default ctor is compiler generated as opposed to user-defined.
• new C - default-initializes C, which calls the default ctor.
• new C() - value-initializes C, which calls the default ctor.

new Thing(); is explicit that you want a constructor called whereas new Thing; is taken to imply you don’t mind if the constructor isn’t called.
new Thing(); is explicit that you want a constructor called whereas new Thing; is taken to imply you don’t mind if the constructor isn’t called.

If used on a struct/class with a user-defined constructor, there is no difference. If called on a trivial struct/class (e.g. struct Thing { int i; };) then new Thing; is like malloc(sizeof(Thing)); whereas new Thing(); is like calloc(sizeof(Thing)); - it gets zero initialized.