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Merge pull request MicrosoftDocs#650 from MicrosoftDocs/cr-permissive-effect
Add /Zc:twoPhase-, effect of /permissive- on /Zc switches
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‎docs/build/reference/TOC.md‎

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##### [/Zc:threadSafeInit (Thread-safe Local Static Initialization)](zc-threadsafeinit-thread-safe-local-static-initialization.md)
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##### [/Zc:throwingNew (Assume operator new throws)](zc-throwingnew-assume-operator-new-throws.md)
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##### [/Zc:trigraphs (Trigraphs Substitution)](zc-trigraphs-trigraphs-substitution.md)
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##### [/Zc:twoPhase- (disable two-phase name lookup)](zc-twophase.md)
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##### [/Zc:wchar_t (wchar_t Is Native Type)](zc-wchar-t-wchar-t-is-native-type.md)
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#### [/Zf (Faster PDB generation)](zf.md)
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#### [/Zg (Generate Function Prototypes)](zg-generate-function-prototypes.md)

‎docs/build/reference/permissive-standards-conformance.md‎

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## Remarks
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You can use the **/permissive-** compiler option to specify standards-conforming compiler behavior. This option disables permissive behaviors, and sets the [/Zc](../../build/reference/zc-conformance.md) compiler options for strict conformance. In the IDE, this option also makes the IntelliSense engine underline non-conforming code.
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You can use the **/permissive-** compiler option to specify standards-conforming compiler behavior. This option disables permissive behaviors, and sets the [/Zc](../../build/reference/zc-conformance.md) compiler options for strict conformance. In the IDE, this option also makes the IntelliSense engine underline non-conforming code.
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By default, the **/permissive-** option is set in new projects created by Visual Studio 2017 version 15.5 and later versions. It is not set by default in earlier versions. When the option is set, the compiler generates diagnostic errors or warnings when non-standard language constructs are detected in your code, including some common bugs in pre-C++11 code.
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The **/permissive-** option is compatible with almost all of the header files from the latest Windows Kits, such as the Software Development Kit (SDK) or Windows Driver Kit (WDK), starting in the Windows Fall Creators
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SDK (10.0.16299.0). Older versions of the SDK may fail to compile under **/permissive-** for various source code conformance reasons. The compiler and SDKs ship on different release timelines, therefore there are some remaining issues. For specific header file issues, see [Windows header issues](#windows-header-issues) below.
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The **/permissive-** option is compatible with almost all of the header files from the latest Windows Kits, such as the Software Development Kit (SDK) or Windows Driver Kit (WDK), starting in the Windows Fall Creators SDK (10.0.16299.0). Older versions of the SDK may fail to compile under **/permissive-** for various source code conformance reasons. The compiler and SDKs ship on different release timelines, therefore there are some remaining issues. For specific header file issues, see [Windows header issues](#windows-header-issues) below.
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The **/permissive-** option sets the [/Zc:strictStrings](../../build/reference/zc-conformance.md) and [/Zc:rvalueCast](../../build/reference/zc-conformance.md) options to conforming behavior. They default to non-conforming behavior. You can pass specific **/Zc** options after **/permissive-** on the command line to override this behavior.
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1. Open your project's **Property Pages** dialog box.
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1. Under **Configuration Properties**, expand the **C/C++** folder and choose the **Command Line** property page.
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1. Select the **Configuration Properties** > **C/C++** > **Command Line** property page.
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1. Enter the **/permissive-** compiler option in the **Additional Options** box. Choose **OK** or **Apply** to save your changes.
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‎docs/build/reference/zc-alignednew.md‎

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---
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title: "/Zc:alignedNew (C++17 over-aligned allocation) | Microsoft Docs"
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ms.date: "12/14/2017"
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ms.date: "02/28/2018"
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ms.technology: ["cpp-tools"]
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Visual Studio version 15.5 enables compiler and library support for C++17 standard over-aligned dynamic memory allocation. When the **/Zc:alignedNew** option is specified, a dynamic allocation such as `new Example;` respects the alignment of *Example* even when it’s greater than `max_align_t`, the largest alignment required for any fundamental type. When the alignment of the allocated type is no more than that guaranteed by the original operator **new**, available as the value of the predefined macro **\_\_STDCPP\_DEFAULT\_NEW\_ALIGNMENT\_\_**, the statement `new Example;` results in a call to `::operator new(size_t)` as it did in C++14. When the alignment is greater than **\_\_STDCPP\_DEFAULT\_NEW\_ALIGNMENT\_\_**, the implementation instead obtains the memory by using `::operator new(size_t, align_val_t)`. Similarly, deletion of over-aligned types invokes `::operator delete(void*, align_val_t)` or the sized delete signature `::operator delete(void*, size_t, align_val_t)`.
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The **/Zc:alignedNew** 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. Under **/std:c++17** or **/std:c++latest**, **/Zc:alignedNew** is enabled by default to conform to the ISO C++17 standard. If the only reason you implement operator **new** and **delete** is to support over-aligned allocations, you may no longer need this code in C++17 mode. To turn this option off and revert to the C++14 behavior of **new** and **delete** when **/std::c++17** or **/std:c++latest** is specified, specify **/Zc:alignedNew-**. If you implement operator **new** and **delete** but you are not ready to implement the over-aligned operator **new** and **delete** overloads that have the `align_val_t` parameter, use the **/Zc:alignedNew-** option to prevent the compiler and Standard Library from generating calls to the over-aligned overloads.
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The **/Zc:alignedNew** 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. Under **/std:c++17** or **/std:c++latest**, **/Zc:alignedNew** is enabled by default to conform to the ISO C++17 standard. If the only reason you implement operator **new** and **delete** is to support over-aligned allocations, you may no longer need this code in C++17 mode. To turn this option off and revert to the C++14 behavior of **new** and **delete** when **/std::c++17** or **/std:c++latest** is specified, specify **/Zc:alignedNew-**. If you implement operator **new** and **delete** but you are not ready to implement the over-aligned operator **new** and **delete** overloads that have the `align_val_t` parameter, use the **/Zc:alignedNew-** option to prevent the compiler and Standard Library from generating calls to the over-aligned overloads. The [/permissive-](permissive-standards-conformance.md) option does not change the default setting of **/Zc:alignedNew**.
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## Example
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1. Open the project's **Property Pages** dialog box. For details, see [Working with Project Properties](../../ide/working-with-project-properties.md).
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1. Select the **Command Line** property page in the **C/C++** folder.
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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:alignedNew** or **/Zc:alignedNew-** and then choose **OK**.
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‎docs/build/reference/zc-auto-deduce-variable-type.md‎

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title: "-Zc:auto (Deduce Variable Type) | Microsoft Docs"
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ms.date: "11/04/2016"
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f1_keywords: ["/Zc:auto"]
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helpviewer_keywords: ["-Zc compiler options (C++)", "Deduce variable type (C++)", "/Zc compiler options (C++)", "Zc compiler options (C++)"]
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---
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# /Zc:auto (Deduce Variable Type)
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The **/Zc:auto[-]** compiler option tells the compiler how to use the [auto keyword](../../cpp/auto-keyword.md) to declare variables. If you specify the default option, **/Zc:auto**, the compiler deduces the type of the declared variable from its initialization expression. If you specify **/Zc:auto-**, the compiler allocates the variable to the automatic storage class.
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## Syntax
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```
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/Zc:auto[-]
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```
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## Remarks
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The C++ standard defines an original and a revised meaning for the `auto` keyword. Before [!INCLUDE[cpp_dev10_long](../../build/includes/cpp_dev10_long_md.md)], the keyword declares a variable in the automatic storage class; that is, a variable that has a local lifetime. Starting with [!INCLUDE[cpp_dev10_long](../../build/includes/cpp_dev10_long_md.md)], the keyword deduces the type of a variable from the declaration's initialization expression. Use the **/Zc:auto[-]** compiler option to tell the compiler to use the original or revised meaning of the `auto` keyword.
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The compiler issues an appropriate diagnostic message if your use of the `auto` keyword contradicts the current compiler option. For more information, see [auto Keyword](../../cpp/auto-keyword.md). For more information about conformance issues with Visual C++, see [Nonstandard Behavior](../../cpp/nonstandard-behavior.md).
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### To set this compiler option in Visual Studio
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1. Open the project's **Property Pages** dialog box. For details, see [Working with Project Properties](../../ide/working-with-project-properties.md).
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2. Click the **Configuration Properties** node.
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3. Click the **C/C++** node.
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4. Click the **Command Line** node.
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## See Also
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[/Zc (Conformance)](../../build/reference/zc-conformance.md)
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[auto Keyword](../../cpp/auto-keyword.md)
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The **/Zc:auto[-]** compiler option tells the compiler how to use the [auto keyword](../../cpp/auto-keyword.md) to declare variables. If you specify the default option, **/Zc:auto**, the compiler deduces the type of the declared variable from its initialization expression. If you specify **/Zc:auto-**, the compiler allocates the variable to the automatic storage class.
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## Syntax
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> **/Zc:auto**[**-**]
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## Remarks
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The C++ standard defines an original and a revised meaning for the `auto` keyword. Before [!INCLUDE[cpp_dev10_long](../../build/includes/cpp_dev10_long_md.md)], the keyword declares a variable in the automatic storage class; that is, a variable that has a local lifetime. Starting with [!INCLUDE[cpp_dev10_long](../../build/includes/cpp_dev10_long_md.md)], the keyword deduces the type of a variable from the declaration's initialization expression. Use the **/Zc:auto[-]** compiler option to tell the compiler to use the original or revised meaning of the `auto` keyword. The **/Zc:auto** option is on by default. The [/permissive-](permissive-standards-conformance.md) option does not change the default setting of **/Zc:auto**.
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The compiler issues an appropriate diagnostic message if your use of the `auto` keyword contradicts the current **/Zc:auto** compiler option. For more information, see [auto Keyword](../../cpp/auto-keyword.md). For more information about conformance issues with Visual C++, see [Nonstandard Behavior](../../cpp/nonstandard-behavior.md).
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### To set this compiler option in Visual Studio
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1. Open the project's **Property Pages** dialog box. For details, see [Working with Project Properties](../../ide/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. Add **/Zc:auto** or **/Zc:auto-** to the **Additional options:** pane.
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## See also
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[/Zc (Conformance)](../../build/reference/zc-conformance.md)<br/>
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[auto Keyword](../../cpp/auto-keyword.md)

‎docs/build/reference/zc-conformance.md‎

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title: "/Zc (Conformance) | Microsoft Docs"
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helpviewer_keywords: ["/Zc compiler options [C++]", "-Zc compiler options [C++]", "Conformance compiler options", "Zc compiler options [C++]"]
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## Syntax
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> **/Zc:**_option_{,_option_}
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When Visual Studio has implemented an extension to C or C++ that is not compatible with the standard, you can use a `/Zc` conformance option to specify standard-conforming or Microsoft-specific behavior. For some options, the Microsoft-specific behavior is the default, to prevent large-scale breaking changes to existing code. In other cases, the default is the standard behavior, where improvements in security, performance, or compatibility outweigh the costs of breaking changes. The default setting of each conformance option may change in newer versions of Visual Studio. For more information about each conformance option, see the topic for the specific option.
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When Visual Studio has implemented an extension to C or C++ that is not compatible with the standard, you can use a `/Zc` conformance option to specify standard-conforming or Microsoft-specific behavior. For some options, the Microsoft-specific behavior is the default, to prevent large-scale breaking changes to existing code. In other cases, the default is the standard behavior, where improvements in security, performance, or compatibility outweigh the costs of breaking changes. The default setting of each conformance option may change in newer versions of Visual Studio. For more information about each conformance option, see the topic for the specific option. The [/permissive-](permissive-standards-conformance.md) compiler option implicitly sets the conformance options that are not set by default to their conformant setting.
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These are the `/Zc` compiler options:
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|[threadSafeInit\[-\]](zc-threadsafeinit-thread-safe-local-static-initialization.md)|Enable thread-safe local static initialization (on by default).|
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|[throwingNew\[-\]](zc-throwingnew-assume-operator-new-throws.md)|Assume `operator new` throws on failure (off by default).|
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|[trigraphs\[-\]](zc-trigraphs-trigraphs-substitution.md)|Enable trigraphs (obsolete, off by default).|
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|[twoPhase-](zc-twophase.md)|Use non-conforming template parsing behavior (conforming by default).|
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|[wchar_t\[-\]](zc-wchar-t-wchar-t-is-native-type.md)|`wchar_t` is a native type, not a typedef (on by default).|
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For more information about conformance issues in Visual C++, see [Nonstandard Behavior](../../cpp/nonstandard-behavior.md).

‎docs/build/reference/zc-externconstexpr.md‎

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title: "/Zc:externConstexpr (Enable extern constexpr variables) | Microsoft Docs"
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The **/Zc:externConstexpr** compiler option causes the compiler to apply external linkage to variables declared by using `extern constexpr`. The **/Zc:externConstexpr** option is available starting in Visual Studio 2017 Update 15.5. In earlier versions of Visual Studio, and by default or if **/Zc:externConstexpr-** is specified, Visual Studio applies internal linkage to `constexpr` variables even if the `extern` keyword is used.
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The **/Zc:externConstexpr** compiler option causes the compiler to apply external linkage to variables declared by using `extern constexpr`. In earlier versions of Visual Studio, and by default or if **/Zc:externConstexpr-** is specified, Visual Studio applies internal linkage to `constexpr` variables even if the `extern` keyword is used. The **/Zc:externConstexpr** option is available starting in Visual Studio 2017 Update 15.6. and is off by default. The [/permissive-](permissive-standards-conformance.md) option does not enable **/Zc:externConstexpr**.
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If a header file contains a variable declared `extern constexpr`, it must be marked [__declspec(selectany)](../../cpp/selectany.md) in order to merge the duplicate declarations into a single instance in the linked binary. Otherwise you may see linker errors, for example, LNK2005, for violations of the one-definition rule.
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### To set this compiler option in Visual Studio
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1. Add **/Zc:externConstexpr** or **/Zc:externConstexpr-** to the **Additional options:** pane.
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