Using namespace header file c++

Question

I'm trying to get a good structure in some code I'm writing but I'm not quite sure about header files. One of the problems I have is: I know I'm not supposed to include namespaces in header files but I don't know where else to put it.

Consider this header:

// deck.h
#pragma once
#include <vector>
#include "card.h"
using namespace std;
typedef vector<card> pile;
class deck{
public:
 deck();
 ~deck();
 void shuffle();
 card takeCard();
 int getSize();
private:
 pile cDeck;
};

The code in card.h is the definition of a card struct. Should the typedef even be in the header file? And ifso, how do I avoid using std::vector?

Any tips would be appriciated.

• πάντα ῥεῖ: using namespace std; in a header file is a pretty bad idea!
• molbdnilo: You shouldn't avoid #include <vector> or std::vector, you need only avoid using namespace std;. (Also, if the pile type is internal to the class, which seems like a good idea, the typedef should be in the private section of the class.)
• iBug: using std::vector is a super better option here!

Answer 1

You don't put it at all. Use std::vector instead of vector, there are only 5 additional characters to type. The typedef is OK to be in header files.

As you probably are aware, using namespace std; in a header is BAD. Why? Because all files that will include your header will automatically use namespace std;, and it's relatively easy to get into conflicting names, especially in large projects where the client may not be aware of the using directive he/she is implicitly using.

Alternatively, you may use namespace std; inside an inline function definition in the header,

inline void f()
{
 using namespace std;
 cout << "bla" << endl;
}

or classes,

class Foo
{
using namespace std;
 // rest
};

This way, the using is effectively "seen" only in its enclosing scope.

Related: What's the scope of the "using" declaration in C++?

• Dirk laren: The 5 characters is true for this header, but I can imagine there are much bigger headers in which it could actually be quite a hassle to type it everytime. So even if the header file would be a bigger one, which would use a certain namespace a lot, you would still use xxx::xx? It seems wrong to have to type it multiple times.
• vsoftco: @Dirklaren I was just editing actually. You can use namespace std; in a scope, so it won't "leak" outside of it.
• Benjamin Lindley: @Dirklaren: Some of us use std:: every time. It's really a normal thing to do. The only time I use a namespace directive is with long nested namespaces, like boost::gregorian. Even then, I don't completely remove it, I just come up with an abbreviation for it (e.g. namespace dt = boost::gregorian;), and only in the scope of a function.
• Lightness Races in Orbit: "So even if the header file would be a bigger one, which would use a certain namespace a lot, you would still use xxx::xx?" Yes "It seems wrong to have to type it multiple times." It's really a tiny proportion of the things I type.

Answer 2

Removing the using directive is trivial, just rewrite the code to

// deck.h
#pragma once
#include <vector>
#include "card.h"
typedef std::vector<card> pile;
class deck{
public:
 deck();
 ~deck();
 void shuffle();
 card takeCard();
 int getSize();
private:
 pile cDeck;
};

Answer 3

Using and using namespace can be used within functions within your implementation. If you want to limit possible conflicts as mentioned above, and you are in a situation where you really need to use "using" or "using namespace", keep it within small functions.