difference between new Test and new Test()
http://stackoverflow.com/questions/620137/do-the-parentheses-after-the-type-name-make-a-difference-with-new
if 'Test' is an ordinary class, is there any difference between:
Test* test = new Test;
//and
Test* test = new Test();
Sometimes the memory returned by the new operator will be initialized, and sometimes it won't depending on whether the type you're newing up is a POD, or if it's a class that contains POD members and is using a compiler-generated default constructor.
- In C++1998 there are 2 types of initialization: zero and default
- In C++2003 a 3rd type of initialization, value initialization was added.
Assume:
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++98 compiler, the following should occur:
- new A - indeterminate value
new A() - zero-initialize
new B - default construct (B::m is uninitialized)
new B() - default construct (B::m is uninitialized)
new C - default construct (C::m is zero-initialized)
- new C() - default construct (C::m is zero-initialized)
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.
So in all versions of C++ there's a difference between "new A" and "new A()" because A is a POD.
And there's a difference in behavior between C++98 and C++03 for the case "new B()".
This is one of the dusty corners of C++ that can drive you crazy. When constructing an object, sometimes you want/need the parens, sometimes you absolutely cannot have them, and sometimes it doesn't matter.
