CODEWITHSUNDEEP
c++classsyntaxnamespacesdefinition
Iraimbilanja
Iraimbilanja

Reputation:

Defining a class within a namespace

Is there a more succinct way to define a class in a namespace than this:

namespace ns { class A {}; }

I was hoping something like class ns::A {}; would work, but alas not.

Upvotes: 29

Views: 62165

Answers (4)

D.Shawley
D.Shawley

Reputation: 59553

You're close, you can forward declare the class in the namespace and then define it outside if you want:

namespace ns {
    class A; // just tell the compiler to expect a class def
}

class ns::A {
    // define here
};

What you cannot do is define the class in the namespace without members and then define the class again outside of the namespace. That violates the One Definition Rule (or somesuch nonsense).

Upvotes: 49

Logan Capaldo
Logan Capaldo

Reputation: 40336

You can do that, but it's not really more succint.

namespace ns {
    class A;
}

class ns::A {
};

Or

namespace ns {
    class B;
}

using ns::B;
class B {
};

Upvotes: 7

dirkgently
dirkgently

Reputation: 111120

The section you should be reading is this:

7.3.1.2 Namespace member definitions

3 Every name first declared in a namespace is a member of that namespace.[...]

Note the term -- declaration so D.Shawley (and his example) is correct.

Upvotes: 2

anon
anon

Reputation:

No you can't. To quote the C++ standard, section 3.3.5:

A name declared outside all named or unnamed namespaces (7.3), blocks (6.3), fun (8.3.5), function definitions (8.4) and classes (clause 9) has global namespace scope

So the declaration must be inside a namespace block - the definition can of course be outside it.

Upvotes: 0

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