There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:
// File1.cs
using System;
namespace Outer.Inner
{
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now imagine that someone adds another file (File2.cs) to the project that looks like this:
// File2.cs
namespace Outer
{
class Math
{
}
}
The compiler searches Outer
before looking at those using
directives outside the namespace, so it finds Outer.Math
instead of System.Math
. Unfortunately (or perhaps fortunately?), Outer.Math
has no PI
member, so File1 is now broken.
This changes if you put the using
inside your namespace declaration, as follows:
// File1b.cs
namespace Outer.Inner
{
using System;
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now the compiler searches System
before searching Outer
, finds System.Math
, and all is well.
Some would argue that Math
might be a bad name for a user-defined class, since there's already one in System
; the point here is just that there is a difference, and it affects the maintainability of your code.
It's also interesting to note what happens if Foo
is in namespace Outer
, rather than Outer.Inner
. In that case, adding Outer.Math
in File2 breaks File1 regardless of where the using
goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using
directive.
The return value for main
indicates how the program exited. Normal exit is represented by a 0 return value from main
. Abnormal exit is signaled by a non-zero return, but there is no standard for how non-zero codes are interpreted. As noted by others, void main()
is prohibited by the C++ standard and should not be used. The valid C++ main
signatures are:
int main()
and
int main(int argc, char* argv[])
which is equivalent to
int main(int argc, char** argv)
It is also worth noting that in C++, int main()
can be left without a return-statement, at which point it defaults to returning 0. This is also true with a C99 program. Whether return 0;
should be omitted or not is open to debate. The range of valid C program main signatures is much greater.
Efficiency is not an issue with the main
function. It can only be entered and left once (marking the program's start and termination) according to the C++ standard. For C, re-entering main()
is allowed, but should be avoided.
Best Answer
Returning null is usually the best idea if you intend to indicate that no data is available.
An empty object implies data has been returned, whereas returning null clearly indicates that nothing has been returned.
Additionally, returning a null will result in a null exception if you attempt to access members in the object, which can be useful for highlighting buggy code - attempting to access a member of nothing makes no sense. Accessing members of an empty object will not fail meaning bugs can go undiscovered.