Re: Are int a; int a(); int a=0; the same?
PengYu.UT@gmail.com wrote:
On Nov 17, 10:03 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:
<PengYu...@gmail.com> wrote in message
news:fa72d4e5-48c2-4307-91d3-a6aa63e5d275@a28g2000hsc.googlegroups.com...
Hi,
I'm wondering if the following three statements are the same or only
the last two are the same?
int a;
int a();
int a=0;
You missed one
int a(0);
The three you show do different things.
The first one declares a as an interger which is unitialized. Can
contain any value. Be careful, some compilers will initialize
variables to 0 in debug mode but not release mode sometimes causing
hard to find errors.
The second one declares a function called a accepting no parameters
and retuning an integer.
The third one declares a as in integer and initializes it to zero.
The fourth one also declares a as an integer and initializes it to
zero.
Note:
int a;
a = 0;
is different than
int a = 0;
The first case uses the assignment operator. The second case uses
the constructor.
My question was actually raised when I want to define a variable of
type T in a template function and I want that variable be initialized
to zero.
For example, let us say the template argument is T, which could be any
numerical classes or basic types, such as, int, double, complex, or
user defined numerical type from third party library, e.g. bigint.
Which one shall I use?
T a = 0;
T a = T();
T a;
Or cause, all classes has a default constructor, then T a; would be OK
^^^^^^^^
"Of course"?
You probably should use "_if_ the class has the default c-tor, then T a;
would be OK".
(suppose all the default constructor would initialize it to zero). But
this is not good for basic types
Correct.
.
T a =0; would be good for basic types, but the classes from a third
party library might not have an constructor like that.
That would require a constructor that takes an integral value or
a pointer and an accessible copy-constructor. Usually they are there,
but sometimes they aren't.
T a; a = 0; has the same problem. The class might not have an
assignment operator.
Right.
In order to make T good for as many situations as possible, what is
the common practice?
The common practice is to require something. In the case
T a = T();
a default c-tor (for classes) and accessible copy c-tor are required.
The built-in types work very well in this case.
Here is a possible scheme that should do the same thing, essentially.
static T zeroT; // will be zero-initialised before anything else
// so it should be OK even for built-in types
T a(zeroT); // needs a copy-ctor
....
V
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