Programming Forums > Application Development > C++

grumpy · Apr 14th, 2006, 5:14 AM

The original question was not that clear, Jimbo.

Some people, when they have nothing to say, point out the irrelevant, eh tempest?

biohazard · Apr 14th, 2006, 5:47 AM

It might appear completely unrelated to you Grumpy, but I am new to namespaces. So I was wondering why you did not use namespace std. Now I got the point. Using std:: exempts one from bringingwhole of namespace into the scope.

grumpy · Apr 14th, 2006, 6:15 AM

Quote:

Originally Posted by biohazard

It might appear completely unrelated to you Grumpy, but I am new to namespaces.

I didn't say your question was unrelated. I simply referred back to my previous post in the process of answering.

Quote:

Originally Posted by biohazard

So I was wondering why you did not use namespace std. Now I got the point. Using std:: exempts one from bringingwhole of namespace into the scope.

What you describe here is the reverse of reality.

A "using namespace" directive does not bring a namespace into scope. It actually tells the compiler that, if it encounters a name, that it should look within the namespace for candidates that match that name.

A "using namespace std;" directive potentially exempts one from having to use the "std::" prefix to fully qualify a name (not the reverse, which you stated). I say potentially, because it can also introduces ambiguity. For example;

(Toggle Plain Text)

#include <iostream>

using namespace std;

namespace X
{
    int cout;
};

using namespace X;

int main()
{
    cout << "Hello\n";
}

will result in a compile error, as there are two candidates named cout (std::cout and X::cout) that are candidates to match the name used in main(), and the compiler has no reason to prefer either one over the other. "using namespace" directives cannot be switched off after the fact, so the only way to eliminate the ambiguity is to change the main() function to this;

(Toggle Plain Text)

int main()
{
    std::cout << "Hello\n";
}

InfoGeek · Apr 14th, 2006, 6:43 AM

Quote:

Originally Posted by InfoGeek

Also I'd bring the whole of the std namespace into scope.

Quote:

Originally Posted by grumpy

A "using namespace" directive does not bring a namespace into scope. It actually tells the compiler that, if it encounters a name, that it should look within the namespace for candidates that match that name.

I stand corrected.

biohazard · Apr 14th, 2006, 7:14 AM

(Toggle Plain Text)

using namespace std;

namespace X
{
    int cout;
};

What is namespace X for, as we are already using namespace std?Is it a namespace in a namespace?

InfoGeek · Apr 14th, 2006, 7:19 AM

It's a user-defined namespace.

EDIT: you may like to read about namespaces here(first google hit).

biohazard · Apr 14th, 2006, 7:46 AM

Thanx InfoGeek. I got a basic idea of namespaces.

biohazard · Apr 14th, 2006, 7:47 AM

Correction: I meant using namespaces. Sorry.

grumpy · Apr 14th, 2006, 10:53 AM

namespace X was a namespace I used in the example to illustrate one of the problems that can be cased by "using namespace" directives. The purpose of namespaces is to allow a program to use multiple things with the same name (eg cout in my example), as long as the things with the same name are placed into different namespaces. A using directive can work against that purpose (and confuse the compiler) if not used carefully.

Apr 14th, 2006, 7:14 AM	#15
biohazard Newbie Join Date: Feb 2006 Posts: 18 Rep Power: 0	(Toggle Plain Text) using namespace std; namespace X { int cout; }; using namespace std; namespace X { int cout; }; What is namespace X for, as we are already using namespace std?Is it a namespace in a namespace?

Apr 14th, 2006, 7:19 AM	#16
InfoGeek Professional Programmer Join Date: Jun 2005 Location: India, The great. Posts: 435 Rep Power: 4	It's a user-defined namespace. EDIT: you may like to read about namespaces here(first google hit). __________________ PFO - My daily dose of technology.

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Apr 14th, 2006, 5:14 AM	#11
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	The original question was not that clear, Jimbo. Some people, when they have nothing to say, point out the irrelevant, eh tempest?

Apr 14th, 2006, 5:47 AM	#12
biohazard Newbie Join Date: Feb 2006 Posts: 18 Rep Power: 0	It might appear completely unrelated to you Grumpy, but I am new to namespaces. So I was wondering why you did not use namespace std. Now I got the point. Using std:: exempts one from bringingwhole of namespace into the scope.

Apr 14th, 2006, 7:46 AM	#17
biohazard Newbie Join Date: Feb 2006 Posts: 18 Rep Power: 0	Thanx InfoGeek. I got a basic idea of namespaces.

Apr 14th, 2006, 7:47 AM	#18
biohazard Newbie Join Date: Feb 2006 Posts: 18 Rep Power: 0	Correction: I meant using namespaces. Sorry.

Apr 14th, 2006, 10:53 AM	#19
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	namespace X was a namespace I used in the example to illustrate one of the problems that can be cased by "using namespace" directives. The purpose of namespaces is to allow a program to use multiple things with the same name (eg cout in my example), as long as the things with the same name are placed into different namespaces. A using directive can work against that purpose (and confuse the compiler) if not used carefully.

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Programming Forums > Application Development > C++

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