why use namespace std?

[FireCoder]

Why do some programmers not use the line of code:

using namespace std;

Should I not use it? Is it just a short cut for beginners?

[yR9wb7Xj]

I think it depends on your preference. I honestly would find it annoying if I had to type std:: to every single line that needed to be there in order to execute the line of code I want. So I prefer to use using namespace std; because it saves me so much time on writing code, and I honestly think it looks a lot better and cleaner.

[helios]

Back in the days of C, there were two solutions to the problem of naming functions:
1. Just name it what you think it should be called and pray that no one else has made a different function with the same name.
2. Follow the convention <project>_<function>.
#1 could create conflicts between unrelated libraries that just happened to have functions named the same, and #2 is a bit ugly.

Namespaces are more elegant solution. using namespace in a very broad scope sends you right back to the days of C. You better not make any functions named max, or find, or sort. Two safer alternatives are:
1. Just use what you will use the most. E.g. using boost::filesystem::path;
2. Restrict the using to a more limited scope. E.g.

void foo(){
    using namespace std;
    cout << "This works.\n";
}

void bar(){
    cout << "This doesn't.\n";
}

Personally, I don't find typing "std::" every time that big a deal. Some Boost namespaces can get rather cumbersome, though.

[liuyang]

It is not recommended to use this line, it will cause namespace pollution. The namespace is used to put related things together to avoid naming conflicts. You should use, for example:

std::cout << "Hello" << std::endl;

or

using std::cout;
using std::endl;

cout << "Hello" << endl;

[keskiverto]

See http://www.gotw.ca/gotw/053.htm

[MiiNiPaa]

http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice
Here are some example where using namepaces at global scope could harm you.
And another example of name clash:

#include <iostream>
#include <algorithm>
using namespace std;
void func1();
void func2();
int count;

int main()
{
    int i;
    for (i=0; i<10; i++) {
        count = i * 2;
        func1();
    }
}
void func1()
{
    func2();
    cout << "count: " << count;
    cout << '\n';
}
void func2()
{
    int count;
    for(count=0; count<3; count++) std::cout<< '.';
}