When Should You Use __try Instead of try/catch/finally in C ?

Exceptions in C : __try vs. try/catch/finally

In C , the try/catch/finally constructs are used to handle exceptions. However, they can also be encountered with underscores, such as __try. This raises the question of when these underscores are necessary.

On Windows Platforms: Unifying Exception Handling

On Windows, exceptions are supported at the operating system level through Structured Exception Handling (SEH). They are comparable to Unix signals. Compilers targeting Windows leverage SEH to implement C exceptions.

Non-Standard __try and __except

To handle SEH exceptions in C , you must use the non-standard __try keyword instead of try. The __except keyword is similar to C 's catch, but it provides additional functionality. It allows you to specify an exception filter expression that determines whether an active exception should be caught.

__finally for Post-Exception Code Execution

The __finally keyword allows you to execute code after an exception has been handled. This feature is not present in standard C , but it is common in other languages.

Disabling Optimization for Destructor Invocation

The Microsoft compiler performs an optimization that can prevent destructors from being invoked in all cases during stack unwinding. If it determines that there is no throw within the scope that governs the object's lifetime, it skips the registration code. To ensure destructor invocation, use the /EHa compile option to suppress this optimization.

Example Demonstrating SEH and C Exception Handling

To illustrate the concepts, here's a code snippet that demonstrates how SEH exceptions allow for C destructor invocation and how C exceptions are built on top of SEH:

#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;
}

Output:

Filtering c0000005
destructed
caught
Filtering e06d7363
destructed
caught

This example showcases the handling of both SEH and C exceptions, demonstrating how destructors are invoked even during SEH exceptions.

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