In practice, the difference is in the location where the preprocessor searches for the included file.
For #include <filename>
the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.
For #include "filename"
the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename>
form. This method is normally used to include programmer-defined header files.
A more complete description is available in the GCC documentation on search paths.
Virtual destructors are useful when you might potentially delete an instance of a derived class through a pointer to base class:
class Base
{
// some virtual methods
};
class Derived : public Base
{
~Derived()
{
// Do some important cleanup
}
};
Here, you'll notice that I didn't declare Base's destructor to be virtual
. Now, let's have a look at the following snippet:
Base *b = new Derived();
// use b
delete b; // Here's the problem!
Since Base's destructor is not virtual
and b
is a Base*
pointing to a Derived
object, delete b
has undefined behaviour:
[In delete b
], if the static type of the
object to be deleted is different from its dynamic type, the static
type shall be a base class of the dynamic type of the object to be
deleted and the static type shall have a virtual destructor or the
behavior is undefined.
In most implementations, the call to the destructor will be resolved like any non-virtual code, meaning that the destructor of the base class will be called but not the one of the derived class, resulting in a resources leak.
To sum up, always make base classes' destructors virtual
when they're meant to be manipulated polymorphically.
If you want to prevent the deletion of an instance through a base class pointer, you can make the base class destructor protected and nonvirtual; by doing so, the compiler won't let you call delete
on a base class pointer.
You can learn more about virtuality and virtual base class destructor in this article from Herb Sutter.
Best Answer
I don't think there's a recommended order, as long as it compiles! What's annoying is when some headers require other headers to be included first... That's a problem with the headers themselves, not with the order of includes.
My personal preference is to go from local to global, each subsection in alphabetical order, i.e.:
My rationale for 1. is that it should prove that each header (for which there is a cpp) can be
#include
d without prerequisites (terminus technicus: header is "self-contained"). And the rest just seems to flow logically from there.