Re: using
Eric Kaplan <tobycraftse@yahoo.com> wrote in
news:g2nbt39907qdnbm5lev6gbqp8uk8m83cgl@4ax.com:
what's difference between using and include?
does using is use for standard library only?
#########################
#include <iostream>
using std::cout;
using std::left;
using std::right;
#include <iomanip>
using std::setw;
#include tells the compiler to start parsing the specified file at this
point. In other words, a textual inclusion of source code.
using on the otherhand is tied up with namespaces. Normally you put
code in a namespace to keep the actual variable/function/class names
separate. This allows you to have a std::copy() and a file::copy() used
in the same source file without conflict. Sometimes you aren't using
more than one of these methods in a file and it can get quite tedious to
always type out the namespace name each time. Now enters the 'using'
keyword. By saying:
using std::copy;
you tell the compiler that when it sees the symbol 'copy' it should use
the one from std. If there are lots of methods from a namespace that
you are using, you can bring the whole namespace into consideration by
saying:
using namespace std;
This means that you can use all the methods/variables/etc from std (or
any other namespace specified in place of std above) without explicitly
qualifying them. Now you can use (assuming you have included iostream,
iomanip etc somewhere prior):
cout << "this is text" << endl;
instead of:
std::cout << "this is text" << std::endl;
Personal style preference will drive your thoughts on whether this is a
good thing to do or not. For example, I would never do:
using file::copy;
because if I see some code that has:
copy(firststring, secondstring);
I would be likely to think that the string was being copied whereas if I
saw:
file::copy(firststring, secondstring);
I would know that a file copy was happening without some context search.
So, the caution is to use the 'using' keyword with a bit of caution.
Another guideline is to never use the using keyword in a header. The
reason for this is that if you have a using in the header, you force all
the names from that namespace into the namespace of the user of your
header file and that is just impolite at best and can cause hard to find
bugs at worst.
HTH,
joe