Re: Being Confused by /EH Exception Handling Model

<wuguangwen734@gmail.com> wrote:


I'm sorry for the confusion I caused. MSDN article indeed mentions
C++ exceptions. I was wrong.


/EHc switch does not control whether an exception is caught or
not. /EHc switch is a kid of optimization for the compiler. If
compiler can assume that extern "C" functions don't throw C++
exception, then it can eliminate some code, which is necessary
otherwise. For example, consider the following simple program:

// ex.cpp

#include <stdio.h>

struct X
{
    X() { puts("X::X"); }
    ~X() { puts("X::~X"); }
};

void f1()
{
    throw 1;
}

extern "C" void f2()
{
    f1();
}

int main()
{
    try
    {
        X x1;
        f2();
    }
    catch(...)
    {
        puts("exception is caught");
    }

    return 0;
}

// end of ex.cpp

Now, when this program compiled with /EHs (that is, the compiler
assumes that extern C functions do throw an exception), then the
output is:

    X::X
    X::~X
    exception is caught

As you can see, the compiler is prepared to the possibility that
f2 can throw, so it ensures that X destructor is called before
catch clause is entered.

When the program is compiled with /EHsc switch (i.e., the compiler
assumes that extern C functions cannot throw), then the output is:

    X::X
    exception is caught

Now the compiler assumed that since f2 won't throw anything it is
safe to optimize away a call to X destructor. Only one call to
X::~X is left by the compiler - at the end of try block. In the
runtime this point has not been reached because f2 broke its
promise and threw an exception.

HTH
Alex