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.

`using namespace std;` in a header file is a pretty bad idea! — πάντα ῥεῖ, May 23 '15 at 16:19
You shouldn't avoid `#include ` 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.) — molbdnilo, May 23 '15 at 16:23

score 1 · Answer 1 · edited May 23 '17 at 11:43

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++?

edited May 23 '17 at 11:43

Community

1
1

answered May 23 '15 at 16:18

vsoftco

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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. – Dirk laren May 23 '15 at 16:26
@Dirklaren I was just editing actually. You can `use namespace std;` in a scope, so it won't "leak" outside of it. – vsoftco May 23 '15 at 16:27
2

@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. – Benjamin Lindley May 23 '15 at 16:32
_"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. – Lightness Races in Orbit Feb 21 '18 at 15:42

score 0 · Answer 2 · answered May 23 '15 at 16:25

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;
};

score 0 · Answer 3 · answered Feb 21 '18 at 15:29

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.

Using namespace header file c++

3 Answers3