unique - Page 176 - Developer IT

C++ custom exceptions: run time performance and passing exceptions from C++ to C

- by skyeagle

I am writing a custom C++ exception class (so I can pass exceptions occuring in C++ to another language via a C API). My initial plan of attack was to proceed as follows: //C++ myClass { public: myClass(); ~myClass(); void foo() // throws myException int foo(const int i, const bool b) // throws myException } * myClassPtr; // C API #ifdef __cplusplus extern "C" { #endif myClassPtr MyClass_New(); void MyClass_Destroy(myClassPtr p); void MyClass_Foo(myClassPtr p); int MyClass_FooBar(myClassPtr p, int i, bool b); #ifdef __cplusplus }; #endif I need a way to be able to pass exceptions thrown in the C++ code to the C side. The information I want to pass to the C side is the following: (a). What (b). Where (c). Simple Stack Trace (just the sequence of error messages in order they occured, no debugging info etc) I want to modify my C API, so that the API functions take a pointer to a struct ExceptionInfo, which will contain any exception info (if an exception occured) before consuming the results of the invocation. This raises two questions: Question 1 1. Implementation of each of the C++ methods exposed in the C API needs to be enclosed in a try/catch statement. The performance implications for this seem quite serious (according to this article): "It is a mistake (with high runtime cost) to use C++ exception handling for events that occur frequently, or for events that are handled near the point of detection." At the same time, I remember reading somewhere in my C++ days, that all though exception handling is expensive, it only becmes expensive when an exception actually occurs. So, which is correct?. what to do?. Is there an alternative way that I can trap errors safely and pass the resulting error info to the C API?. Or is this a minor consideration (the article after all, is quite old, and hardware have improved a bit since then). Question 2 I wuld like to modify the exception class given in that article, so that it contains a simple stack trace, and I need some help doing that. Again, in order to make the exception class 'lightweight', I think its a good idea not to include any STL classes, like string or vector (good idea/bad idea?). Which potentially leaves me with a fixed length C string (char*) which will be stack allocated. So I can maybe just keep appending messages (delimted by a unique separator [up to maximum length of buffer])... Its been a while since I did any serious C++ coding, and I will be grateful for the help. BTW, this is what I have come up with so far (I am intentionally, not deriving from std::exception because of the performance reasons mentioned in the article, and I am instead, throwing an integral exception (based on an exception enumeration): class fast_exception { public: fast_exception(int what, char const* file=0, int line=0) : what_(what), line_(line), file_(file) {/*empty*/} int what() const { return what_; } int line() const { return line_; } char const* file() const { return file_; } private: int what_; int line_; char const[MAX_BUFFER_SIZE] file_; }

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