[hidden.friends]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#hidden.friends)

### 16.4.6 Conforming implementations [[conforming]](conforming#hidden.friends)

#### 16.4.6.6 Friend functions [hidden.friends]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3709)

Whenever this document specifies
a friend declaration of a function or function template
within a class or class template definition,
that declaration shall be
the only declaration of that function or function template
provided by an implementation[.](#1.sentence-1)

[*Note [1](#note-1)*:

In particular,
a conforming implementation does not provide
any additional declarations of that function or function template
at namespace scope[.](#1.sentence-2)

— *end note*]

[*Note [2](#note-2)*:

Such a friend function or function template declaration
is known as a hidden friend,
as it is visible neither
to ordinary unqualified lookup ([[basic.lookup.unqual]](basic.lookup.unqual "6.5.3 Unqualified name lookup")) nor
to qualified lookup ([[basic.lookup.qual]](basic.lookup.qual "6.5.5 Qualified name lookup"))[.](#1.sentence-3)

— *end note*]