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### 7.10 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.