Let's get pedantic, because there are differences that can actually affect your code's behavior. Much of the following is taken from comments made to an Old New Thing article
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 (plain old data), 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.
I usually try to phrase things differently to differentiate between the two a bit better, but it's a good question in any case.
Operator new is a function that allocates raw memory -- at least conceptually, it's not much different from malloc()
. Though it's fairly unusual unless you're writing something like your own container, you can call operator new directly, like:
char *x = static_cast<char *>(operator new(100));
It's also possible to overload operator new either globally, or for a specific class. IIRC, the signature is:
void *operator new(size_t);
Of course, if you overload an operator new (either global or for a class), you'll also want/need to overload the matching operator delete as well. For what it's worth, there's also a separate operator new[] that's used to allocate memory for arrays -- but you're almost certainly better off ignoring that whole mess completely.
The new operator is what you normally use to create an object from the free store:
my_class *x = new my_class(0);
The difference between the two is that operator new just allocates raw memory, nothing else. The new operator starts by using operator new to allocate memory, but then it invokes the constructor for the right type of object, so the result is a real live object created in that memory. If that object contains any other objects (either embedded or as base classes) those constructors as invoked as well.
Best Answer
There might be difference in contexts involving
std::initializer_list<>
, e.g.:Case 1 -
()
and{}
Case 2:
(v)
and{v}
As stated by Meyers and others there is also a huge difference when using STL:
and it is not restricted to
std::vector
onlyIn this case there's no difference though (and
initializer_list
ctor is ignored)There is also a well-known difference in this case
where
test
is default-initialized object of typeTest
andtest2
is a function declaration.