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c++c++03argument-dependent-lookup
Jan Hudec
Jan Hudec

Reputation: 76386

Argument dependent lookup for classes in top-level namespace?

I have a class that is in top-level namespace for historical reasons. I need to define a function for it that must1 be found using argument-dependent lookup. Is the top-level namespace considered associated namespace of such class or do I have to make some workaround?

1Otherwise the template that uses it might not find it, because symbols defined after the template are only seen when found by ADL.

Upvotes: 1

Views: 111

Answers (2)

Ivan Vergiliev
Ivan Vergiliev

Reputation: 3841

Here is my take on the standard:

From [basic.lookup.argdep] (2):

— If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the innermost enclosing namespaces of its associated classes.

From [basic.namespace] (2) (and 3.3.6 where global namespace is defined):

The outermost declarative region of a translation unit is a namespace; see 3.3.6.

So I don't see anything to exclude the global namespace from ADL.

Upvotes: 1

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171177

The global namespace is not special in this regard, it works just like any other namespace and is thus perfectly fine for your use case.

Live example 1 of global namespace working.

Live example 2 of ADL actually being the reason #1 works1.

Here's the text of the live example:

#include <iostream>

struct X {};

template <class T>
void bar(T t)
{
  foo(t);
}

void foo(X x)
{
  std::cout << "Foo\n";
}

int main()
{
  X x;
  bar(x);
}

1 This fails, because ADL cannot find the function, while normal lookup would. So it proves that #1 really works thanks to ADL.

Upvotes: 2

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