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Add documentation for experimental warning: C6389 #3187

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Add documentation for experimental warning: C6389
Xazax-hun Jun 9, 2021
e0e0429
Add toc.
Xazax-hun Jun 9, 2021
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49 changes: 49 additions & 0 deletions docs/code-quality/c6389.md
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---
title: C6389
description: "Describes the Microsoft C/C++ code analysis warning C6389, its causes, and how to address it."
ms.date: 06/09/2021
f1_keywords: ["C6389"]
helpviewer_keywords: ["C6389"]
---

# C6389: MARK_INTERNAL_OR_MISSING_COMMON_DECL

This check intended to help with reducing the visibility of certain symbols and modularize the code. In multi-file C++ projects each declaration should be either local to a C++ file (part of the anonymous namespace) or declared in a common header file that is included by multiple C++ files.

When this check flags a declaration, either it should be moved to an anonymous namespace or a forward declaration should be moved to a header file depending on the scope of the symbol.

This is an experimental rule that needs to be enabled in a ruleset file explicitly to work. More information about rulesets can be found [here](https://docs.microsoft.com/en-us/visualstudio/code-quality/using-rule-sets-to-group-code-analysis-rules).

## Example
```cpp
// A.h
struct X;

// A.cpp

// Not flagged, declared in a header file.
struct X { int x; };

struct Y { double y; }; // warning: Move 'Y' to anonymous namespace or put a forward declaration in a common header included in this file.

void f(); // warning: Move 'Y' to anonymous namespace or put a forward declaration in a common header included in this file.
```
To resolve this issue:
```cpp
// A.h
struct X;
void f();

// A.cpp

// Not flagged, declared in a header file.
struct X { int x; };

namespace {
struct Y { double y; };
} // anonymous namespace

// Not flagged, declared in a header file.
void f();

```
2 changes: 2 additions & 0 deletions docs/code-quality/toc.yml
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href: ../code-quality/c6387.md
- name: C6388
href: ../code-quality/c6388.md
- name: C6389
href: ../code-quality/c6389.md
- name: C6400
href: ../code-quality/c6400.md
- name: C6401
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