PRB: Unresolved Externals for Inline Functions (123768)

SYMPTOMS

Calling an inline function defined in a separate source file generates the following errors in Visual C++ 32-bit Edition, versions 2.x and 4.0:

   error LNK2001: unresolved external symbol "<function name>"
   <filename.exe> : error LNK1120: 1 unresolved externals

The 32-bit edition of Visual C++ version 1.0 generates these errors:

   warning LNK4016: unresolved external symbol
                     "<Classname>::<Functionname>"
   error LNK1120: 1 unresolved externals

The 16-bit editions generate this error:

   error L2029: '<ClassName>::<FunctionName>' : unresolved external

The function is declared in a header (.H) file, defined as an inline function in one source (.CPP) file, and called from a second source (.CPP) file.

CAUSE

Inline functions are not visible outside of the source file where they are defined. The Inline Specifier section in the C++ Language Reference states, "Functions that are declared as inline and that are not class member functions have internal linkage unless otherwise specified."

The Inline Class Member Functions section in the C++ Language Reference states, "A class member function defaults to external linkage unless a definition for that function contains the inline specifier." Query on <inline function linkage> in the Books On-line for further information.

RESOLUTION

Here are five possible workarounds:

Declare the inline function with the extern keyword to force external linkage. Note, however, that using extern with class member functions will cause a warning with /Ze, and an error with /Za. -or-
Move the function definition to the header file so it's included in each translation unit used. -or-
Remove the inline specification. -or-
Force a function version of the inline function to be created in the module that it is defined in. You can do this by referencing the address of the inline function, by assigning it to a function pointer, or by passing it to a function. -or-
Remove the /Ob1 (Only Inline) or /Ob2 (Any suitable) option from the command line and compile with /Ob0 (Disable). The /Ob0 option is the default if no options are specified.

To change these options from within the Visual Workbench for Visual C++ version 4.0, follow these five steps:
1. Select the Settings... option from the Build menu.
2. Select the C/C+ Tab in the Project Settings dialog box.
3. Select the Optimizations item in the Category drop-down list box.
4. Select one of the following items from the In-line function expansion drop-down list box:
```
         - Disable *  (/Ob0)
         - Only __inline  (/Ob1)
         - Any Suitable  (/Ob2)
								
```
5. Press OK in the Project Settings dialog box.
To change these options from within the Visual Workbench for Visual C++ version 2.x, follow these five steps:
1. Select Project.
2. Choose Settings.
3. Change to the Settings for C/C++.
4. Select the Optimizing Category.
5. Choose the inline option from the Inline Function Expansion box.
To change these options from within previous versions of the Visual Workbench, select Options and choose Project. Then choose Compiler. Change the Category to Optimizations, and choose the inline option from the Inline Expansion of Functions box.

MORE INFORMATION

Sample Code

   /* Compile options needed: /Ob1 or /Ob2
   */ 

   /* TEST.H */ 
   class ClassA
   {
       private:
           int  Var;
       public:
          ClassA ();
          ~ClassA ();
   };

   /* TEST1.CPP */ 
   #include "test.h"
   void testfunc(void);
   inline ClassA::ClassA() { Var = 6; };  // Inline function definition
   ClassA::~ClassA() {}
   void main(void) {
       ClassA test1;
       testfunc();
   }

   /* TEST2.CPP */ 
   #include "test.h"
   void testfunc(void)
   {
       ClassA test2;  // This generates an unresolved external error
                      // on the default constructor when linked
   }