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/Zc:inline (Remove unreferenced COMDAT)

Visual Studio 2013

Removes unreferenced functions or data that are COMDATs or only have internal linkage. When /Zc:inline is specified, the compiler requires that translation units that use inline data or inline functions must also include the definitions for the data or functions.

/Zc:inline[-]

When /Zc:inline is specified, the compiler does not emit symbol information for unreferenced COMDAT functions or data, or for functions or data that have internal linkage only. By default, this option is off (/Zc:inline-). This optimization simplifies some of the work performed by the linker in release builds or when the linker option /OPT:REF is specified. When the compiler performs this optimization, it can significantly reduce .obj file size and improve linker speeds. This compiler option is not enabled when optimizations are disabled (/Od) or when /GL (Whole Program Optimization) is specified.

If /Zc:inline is specified, the compiler enforces the C++11 requirement that all functions declared inline must have a definition available in the same translation unit if they are used. When the option is not specified, Visual C++ allows non-conformant code that invokes functions declared inline even if no definition is visible. For more information, see the C++11 standard, in section 3.2 and section 7.1.2. This compiler option was introduced in Visual Studio 2013 Update 2.

To use the /Zc:inline option, update non-compliant code. This example shows how the non-compliant use of an inline function declaration without a definition still compiles and links when the default /Zc:inline- option is used:

// example.h
// Compile by using: cl /W4 /EHsc /O2 zcinline.cpp example.cpp
#pragma once

class Example {
public:
   inline void inline_call(); // declared but not defined inline
   void normal_call();
   Example() {};
};
// example.cpp
// Compile by using: cl /W4 /EHsc /O2 zcinline.cpp example.cpp
#include <stdio.h>
#include "example.h"

void Example::inline_call() {
   printf("inline_call was called.\n"); 
}

void Example::normal_call() {
   printf("normal_call was called.\n"); 
   inline_call(); // with /Zc:inline-, inline_call forced into .obj file
}
// zcinline.cpp
// Compile by using: cl /W4 /EHsc /O2 zcinline.cpp example.cpp
#include "example.h"

void main() {
   Example example;
   example.inline_call(); // normal call when definition unavailable
}

When /Zc:inline is enabled, the same code causes a LNK2019 error, because the compiler does not emit a non-inlined code body for Example::inline_call in example.obj. This causes the non-inlined call in main to reference an undefined external symbol.

To resolve this error, you can remove the inline keyword from the declaration of Example::inline_call, move the definition of Example::inline_call into the header file, or move the implementation of Example into main.cpp. The next example moves the definition into the header file, where it is visible to any caller that includes the header.

// example2.h
// Compile by using: cl /W4 /EHsc /O2 zcinline2.cpp example2.cpp
#pragma once
#include <stdio.h>

class Example2 {
public:
   inline void inline_call() {
      printf("inline_call was called.\n"); 
   }
   void normal_call();
   Example2() {};
};
// example2.cpp
// Compile by using: cl /W4 /EHsc /O2 zcinline2.cpp example2.cpp
#include "example2.h"

void Example2::normal_call() {
   printf("normal_call was called.\n"); 
   inline_call(); 
}
// zcinline2.cpp
// Compile by using: cl /W4 /EHsc /O2 zcinline2.cpp example2.cpp
#include "example2.h"

void main() {
   Example2 example2;
   example2.inline_call(); // normal call when definition unavailable
}

For more information about conformance issues in Visual C++, see Nonstandard Behavior.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Working with Project Properties.

  2. Select the C/C++ folder.

  3. Select the Command Line property page.

  4. Modify the Additional Options property to include /Zc:inline and then choose OK.

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