When Should You Use `__try` and `__except` Instead of `try` and `catch` in C ?

__try and try/catch/finally in C

In C , exception handling is primarily achieved through the try/catch/finally blocks. However, there are lesser-known commands that start with double underscores, such as __try. This article aims to clarify when these underscores are necessary.

On Windows systems, exceptions are supported through Structured Exception Handling (SEH), an operating system-level mechanism. Compilers typically utilize this SEH infrastructure for C exception implementation. The throw and catch keywords solely handle C exceptions, with their corresponding SEH exception code being 0xe06d7363.

To handle SEH exceptions, C programs require the use of the non-standard __try keyword. Additionally, __except is similar to catch but allows for specifying an exception filter to determine if an active exception should be handled. __finally enables code execution after exception handling.

To illustrate these concepts, consider the example program below:

#include <windows.h>
#include <iostream>

class Example {
public:
    ~Example() { std::cout << "destructed" << std::endl; }
};

int filterException(int code, PEXCEPTION_POINTERS ex) {
    std::cout << "Filtering " << std::hex << code << std::endl;
    return EXCEPTION_EXECUTE_HANDLER;
}

void testProcessorFault() {
    Example e;
    int* p = 0;
    *p = 42;
}

void testCppException() {
    Example e;
    throw 42;
}

int main()
{
    __try {
        testProcessorFault();
    }
    __except(filterException(GetExceptionCode(), GetExceptionInformation())) {
        std::cout << "caught" << std::endl;
    }
    __try {
        testCppException();
    }
    __except(filterException(GetExceptionCode(), GetExceptionInformation())) {
        std::cout << "caught" << std::endl;
    }
    return 0;
}

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