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[CodingStandard] Rework anonymous namespace section to cover visibility more broadly #126775

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merged 4 commits into from
Feb 24, 2025
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[CodingStandard] Rework anonymous namespace section to cover visibility more broadly #126775

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8e9d2e7
[CodingStandard] Add a rule about non-member definitions in CPP files
jurahul Feb 11, 2025
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Update llvm/docs/CodingStandards.rst
jurahul Feb 21, 2025
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Update llvm/docs/CodingStandards.rst
jurahul Feb 21, 2025
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Update llvm/docs/CodingStandards.rst
jurahul Feb 21, 2025
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28 changes: 17 additions & 11 deletions llvm/docs/CodingStandards.rst
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Expand Up @@ -1579,17 +1579,23 @@ clarification.

.. _static:

Anonymous Namespaces
^^^^^^^^^^^^^^^^^^^^

After talking about namespaces in general, you may be wondering about anonymous
namespaces in particular. Anonymous namespaces are a great language feature
that tells the C++ compiler that the contents of the namespace are only visible
within the current translation unit, allowing more aggressive optimization and
eliminating the possibility of symbol name collisions. Anonymous namespaces are
to C++ as "static" is to C functions and global variables. While "``static``"
is available in C++, anonymous namespaces are more general: they can make entire
classes private to a file.
Restrict Visibility
^^^^^^^^^^^^^^^^^^^

Functions and variables should have the most restricted visibility possible.
For class members, that means using appropriate `private`, `protected` or `public`
keyword to restrict their access. For non-member functions, variables and classes,
that means restricting visibility to a single `.cpp` file if it's not referenced
outside that file.

Visibility of file-scoped non-members variables and functions can be restricted to
the current translation unit by using either `static` keyword or anonymous namespace.
Anonymous namespaces are a great language feature that tells the C++ compiler that
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@shafik shafik Mar 3, 2025

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the great here feels a bit like editorializing to me, while I don't disagree it kind of stands out wrt to the rest of the paragraph. I wouldn't bother changing it but for future reference.

the contents of the namespace are only visible within the current translation unit,
allowing more aggressive optimization and eliminating the possibility of symbol
name collisions. Anonymous namespaces are to C++ as `static` is to C functions and
global variables. While `static` is available in C++, anonymous namespaces are more
general: they can make entire classes private to a file.

The problem with anonymous namespaces is that they naturally want to encourage
indentation of their body, and they reduce locality of reference: if you see a
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