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### 6.9 Java Exceptions

The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:

           struct S { ~S(); };
           extern void bar();    // is written in Java, and may throw exceptions
           void foo()
           {
             S s;
             bar();
           }

The usual effect of an incorrect guess is a link failure, complaining of a missing routine called ‘`__gxx_personality_v0`’.

You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing ‘`#pragma GCC java_exceptions`’ at the head of the file. This ‘`#pragma`’ must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.

You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.