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‎docs/build/reference/zc-hiddenfriend.md‎

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---
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title: "/Zc:hiddenFriend (Enforce hidden friend rules)"
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description: "Learn about the Microsoft C++ /Zc:hiddenFriend compiler option for conforming or relaxed hidden friend compatibility."
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ms.date: "06/04/2020"
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f1_keywords: ["/Zc:hiddenFriend"]
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helpviewer_keywords: ["/Zc:hiddenFriend", "Zc:hiddenFriend", "-Zc:hiddenFriend"]
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ms.assetid: 1cbf7e3c-0f82-4f91-84dd-612bcf26d2c6
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---
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# /Zc:hiddenFriend (Enforce hidden friend rules)
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A *hidden friend* is a friend declaration of a function or function template within a class or
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class template definition, that declaration shall be the only declaration of that function or function template
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provided by an implementation.
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The C++20 standard . This change can cause a number of source compatibility issues in code that conforms to C++14 or earlier. The **`/Zc:hiddenFriend`** option specifies conformance to the C++17 standard. **`/Zc:hiddenFriend-`** allows the C++14 and earlier behavior when code is compiled in C++17 mode.
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## Syntax
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> **`/Zc:hiddenFriend`**\[**`-`**]
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## Remarks
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When the **`/Zc:hiddenFriend`** option is specified, the compiler conforms to the C++17 standard and treats [`throw()`](../../cpp/exception-specifications-throw-cpp.md) as an alias for [`noexcept`](../../cpp/noexcept-cpp.md), removes `throw(`*`type-list`*`)` and `throw(...)`, and allows certain types to include **`noexcept`**. The **`/Zc:hiddenFriend`** option is only available when [`/std:c++17`](std-specify-language-standard-version.md) or [`/std:c++latest`](std-specify-language-standard-version.md) is enabled. **`/Zc:hiddenFriend`** is enabled by default to conform to the ISO C++17 standard. The [`/permissive-`](permissive-standards-conformance.md) option doesn't affect **`/Zc:hiddenFriend`**. Turn off this option by specifying **`/Zc:hiddenFriend-`** to revert to the C++14 behavior of **`noexcept`** when **`/std:c++17`** or **`/std:c++latest`** is specified.
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Beginning in Visual Studio 2017 version 15.5, the C++ compiler diagnoses more mismatched exception specifications in declarations in C++17 mode, or when you specify the [`/permissive-`](permissive-standards-conformance.md) option.
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This sample shows how declarations with an exception specifier behave when the **`/Zc:hiddenFriend`** option is set or disabled. To show the behavior when set, compile by using `cl /EHsc /W4 hiddenFriend.cpp`. To show the behavior when disabled, compile by using `cl /EHsc /W4 /Zc:hiddenFriend- hiddenFriend.cpp`.
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```cpp
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// hiddenFriend.cpp
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// Compile by using: cl /EHsc /W4 hiddenFriend.cpp
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// Compile by using: cl /EHsc /W4 /Zc:hiddenFriend- hiddenFriend.cpp
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void f() throw(); // equivalent to void f() noexcept;
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void f() { } // warning C5043
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void g() throw(...); // warning C5040
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struct A
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{
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virtual void f() throw();
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};
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struct B : A
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{
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virtual void f() { } // error C2694
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};
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```
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When compiled by using the default setting **`/Zc:hiddenFriend`**, the sample generates the listed warnings. To update your code, use the following instead:
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```cpp
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void f() noexcept;
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void f() noexcept { }
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void g() noexcept(false);
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struct A
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{
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virtual void f() noexcept;
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};
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struct B : A
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{
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virtual void f() noexcept { }
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};
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```
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For more information about conformance issues in Visual C++, see [Nonstandard Behavior](../../cpp/nonstandard-behavior.md).
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### To set this compiler option in the Visual Studio development environment
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1. Open the project's **Property Pages** dialog box. For details, see [Set C++ compiler and build properties in Visual Studio](../working-with-project-properties.md).
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1. Select the **Configuration Properties** > **C/C++** > **Command Line** property page.
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1. Modify the **Additional Options** property to include *`/Zc:hiddenFriend`* or *`/Zc:hiddenFriend-`* and then choose **OK**.
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## See also
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[**`/Zc`** (Conformance)](zc-conformance.md)\
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[noexcept](../../cpp/noexcept-cpp.md)\
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[Exception specifications (throw)](../../cpp/exception-specifications-throw-cpp.md)

‎docs/build/reference/zc-noexcepttypes.md‎

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## See also
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[**`/Zc`** (Conformance)](zc-conformance.md)\
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[`/Zc` (Conformance)](zc-conformance.md)\
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[noexcept](../../cpp/noexcept-cpp.md)\
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[Exception specifications (throw)](../../cpp/exception-specifications-throw-cpp.md)

‎docs/overview/cpp-conformance-improvements.md‎

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<!-- TODO 16.8 -->
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## <a name="improvements_168"></a> Conformance improvements in Visual Studio 2019 version 16.8
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### 'Class rvalue used as lvalue' extension
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### 'Explicit specialization in non-namespace scope' extension
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### Checking for abstract class types
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### Support for P0960R3 - allow initializing aggregates from a parenthesized list of values
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### Overload resolution involving function templates
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### Migrating from `/await` to C++20 coroutines
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### `/permissive-` and C++20 Modules are on by default under `/std:c++latest`
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C++20 Modules is now supported, by default, under **`/std:c++latest`**. For more details regarding this change and scenarios where module and import are conditionally treated as keywords, see [Standard C++20 Modules support with MSVC in Visual Studio 2019 version 16.8](https://devblogs.microsoft.com/cppblog/standard-c20-modules-support-with-msvc-in-visual-studio-2019-version-16-8/).
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C++20 Modules support is on by default under **`/std:c++latest`**. For more information about this change, and the scenarios where **`module`** and **`import`** are conditionally treated as keywords, see [Standard C++20 Modules support with MSVC in Visual Studio 2019 version 16.8](https://devblogs.microsoft.com/cppblog/standard-c20-modules-support-with-msvc-in-visual-studio-2019-version-16-8/).
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As a prerequisite for Modules support, **`/permissive-`** is now enabled when **`/std:c++latest`** is specified. For more information, see [`/permissive-`](../build/reference/permissive-standards-conformance.md).
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For code that previously compiled under **`/std:c++latest`** and requires non-conformant compiler behaviors, /permissive may be specified to turn off strict conformance mode in the compiler and must appear after **`/std:c++latest`** in the command line argument list. However, specifying /permissive will result in an error if Modules usage is encountered.
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As a prerequisite for Modules support, **`permissive-`** is now enabled when **`/std:c++latest`** is specified. For more information, see [`/permissive-`](../build/reference/permissive-standards-conformance.md).
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error C1214: Modules conflict with non-standard behavior requested via '[option]'
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Where the most common values for [option] are:
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For code that previously compiled under **`/std:c++latest`** and requires non-conforming compiler behaviors, **`permissive`** may be specified to turn off strict conformance mode in the compiler. The compiler option must appear after **`/std:c++latest`** in the command-line argument list. However, **`permissive`** results in an error if Modules usage is encountered:
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Option Description
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/Zc:twoPhase- Two Phase name lookup (/Zc:twoPhase) is required for C++20 Modules and implied by /permissive-. See also /Zc:twoPhase documentation .
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/Zc:hiddenFriend- Enables standard hidden friend name lookup rules is required for C++20 Modules and implied by /permissive-.
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/Zc:preprocessor- Use of the conformant preprocessor (/Zc:preprocessor) is required for C++20 header unit usage and creation only. Named Modules do not require this option. See also /Zc:preprocessor documentation
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When using the std.* Modules which ship with Visual Studio the option /experimental:module is still required for their use as they are not yet standardized.
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> error C1214: Modules conflict with non-standard behavior requested via '*option*'
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Using /experimental:module also implies /Zc:twoPhase and /Zc:hiddenFriend. Previously code compiled with Modules could sometimes be compiled with /Zc:twoPhase- if the Module was only consumed, this behavior is no longer supported.
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### 'Class rvalue used as lvalue' extension
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### 'Class rvalue used as lvalue' extension
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### 'Class rvalue used as lvalue' extension
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### 'Class rvalue used as lvalue' extension
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### 'Class rvalue used as lvalue' extension
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### 'Class rvalue used as lvalue' extension
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The most common values for *option* are:
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| Option | Description |
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|--|--|
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| [`/Zc:twoPhase-`](../build/reference/zc-twophase.md) | Two Phase name lookup is required for C++20 Modules and implied by **`permissive-`**. |
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| [`/Zc:hiddenFriend-`](../build/reference/zc-hiddenfriend.md) | Enables standard hidden friend name lookup rules. Required for C++20 Modules and implied by **`permissive-`**. |
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| [`/Zc:preprocessor-`](../build/reference/zc-preprocessor.md) | The conforming preprocessor is required for C++20 header unit usage and creation only. Named Modules don't require this option. |
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The [`/experimental:module`](../build/reference/experimental-module.md) option is still required to use the *`std.*`* Modules that ship with Visual Studio, because they're not standardized yet.
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The **`/experimental:module`** option also implies **`/Zc:twoPhase`** and **`/Zc:hiddenFriend`**. Previously, code compiled with Modules could sometimes be compiled with **`/Zc:twoPhase-`** if the Module was only consumed. This behavior is no longer supported.
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## <a name="update_160"></a> Bug fixes and behavior changes in Visual Studio 2019
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