try-except statement

The try-except statement is a Microsoft extension that supports structured exception handling in the C and C++ languages. This extension is Microsoft-specific.

Syntax

__try
{
    // guarded code
}
__except ( expression )
{
    // exception handler code
}

Remarks

The try-except statement is a Microsoft extension to the C and C++ languages. It enables target applications to gain control when events occur that normally terminate program execution. Such events are called structured exceptions, or exceptions for short. The mechanism that deals with these exceptions is called structured exception handling (SEH).

For related information, see the try-finally statement.

Exceptions may be either hardware-based or software-based. Structured exception handling is useful even when applications can't completely recover from hardware or software exceptions. SEH makes it possible to display error information and trap the internal state of the application to help diagnose the problem. It's especially useful for intermittent problems that aren't easy to reproduce.

Note

Structured exception handling works with Win32 for both C and C++ source files. However, it's not specifically designed for C++. You can ensure that your code is more portable by using C++ exception handling. Also, C++ exception handling is more flexible, in that it can handle exceptions of any type. For C++ programs, we recommend you use native C++ exception-handling: try, catch, and throw statements.

The compound statement after the __try clause is the body or guarded section. The __except expression is also known as the filter expression. Its value determines how the exception is handled. The compound statement after the __except clause is the exception handler. The handler specifies the actions to take if an exception is raised during execution of the body section. Execution proceeds as follows:

  1. The guarded section is executed.

  2. If no exception occurs during execution of the guarded section, execution continues at the statement after the __except clause.

  3. If an exception occurs during execution of the guarded section, or in any routine the guarded section calls, the __except expression is evaluated. There are three possible values:

    • EXCEPTION_CONTINUE_EXECUTION (-1) Exception is dismissed. Continue execution at the point where the exception occurred.

    • EXCEPTION_CONTINUE_SEARCH (0) Exception isn't recognized. Continue to search up the stack for a handler, first for containing try-except statements, then for handlers with the next highest precedence.

    • EXCEPTION_EXECUTE_HANDLER (1) Exception is recognized. Transfer control to the exception handler by executing the __except compound statement, then continue execution after the __except block.

The __except expression is evaluated as a C expression. It's limited to a single value, the conditional-expression operator, or the comma operator. If more extensive processing is required, the expression can call a routine that returns one of the three values listed above.

Each application can have its own exception handler.

It's not valid to jump into a __try statement, but valid to jump out of one. The exception handler isn't called if a process is terminated in the middle of executing a try-except statement.

For compatibility with previous versions, _try, _except, and _leave are synonyms for __try, __except, and __leave unless compiler option /Za (Disable language extensions) is specified.

The __leave Keyword

The __leave keyword is valid only within the guarded section of a try-except statement, and its effect is to jump to the end of the guarded section. Execution continues at the first statement after the exception handler.

A goto statement can also jump out of the guarded section, and it doesn't degrade performance as it does in a try-finally statement. That's because stack unwinding doesn't occur. However, we recommend that you use the __leave keyword rather than a goto statement. The reason is because you're less likely to make a programming mistake if the guarded section is large or complex.

Structured Exception Handling Intrinsic Functions

Structured exception handling provides two intrinsic functions that are available to use with the try-except statement: GetExceptionCode and GetExceptionInformation.

GetExceptionCode returns the code (a 32-bit integer) of the exception.

The intrinsic function GetExceptionInformation returns a pointer to an EXCEPTION_POINTERS structure containing additional information about the exception. Through this pointer, you can access the machine state that existed at the time of a hardware exception. The structure is as follows:

typedef struct _EXCEPTION_POINTERS {
    PEXCEPTION_RECORD ExceptionRecord;
    PCONTEXT ContextRecord;
} EXCEPTION_POINTERS, *PEXCEPTION_POINTERS;

The pointer types PEXCEPTION_RECORD and PCONTEXT are defined in the include file <winnt.h>, and _EXCEPTION_RECORD and _CONTEXT are defined in the include file <excpt.h>

You can use GetExceptionCode within the exception handler. However, you can use GetExceptionInformation only within the exception filter expression. The information it points to is generally on the stack and is no longer available when control gets transferred to the exception handler.

The intrinsic function AbnormalTermination is available within a termination handler. It returns 0 if the body of the try-finally statement terminates sequentially. In all other cases, it returns 1.

<excpt.h> defines some alternate names for these intrinsics:

GetExceptionCode is equivalent to _exception_code

GetExceptionInformation is equivalent to _exception_info

AbnormalTermination is equivalent to _abnormal_termination

Example

// exceptions_try_except_Statement.cpp
// Example of try-except and try-finally statements
#include <stdio.h>
#include <windows.h> // for EXCEPTION_ACCESS_VIOLATION
#include <excpt.h>

int filter(unsigned int code, struct _EXCEPTION_POINTERS *ep)
{
    puts("in filter.");
    if (code == EXCEPTION_ACCESS_VIOLATION)
    {
        puts("caught AV as expected.");
        return EXCEPTION_EXECUTE_HANDLER;
    }
    else
    {
        puts("didn't catch AV, unexpected.");
        return EXCEPTION_CONTINUE_SEARCH;
    };
}

int main()
{
    int* p = 0x00000000;   // pointer to NULL
    puts("hello");
    __try
    {
        puts("in try");
        __try
        {
            puts("in try");
            *p = 13;    // causes an access violation exception;
        }
        __finally
        {
            puts("in finally. termination: ");
            puts(AbnormalTermination() ? "\tabnormal" : "\tnormal");
        }
    }
    __except(filter(GetExceptionCode(), GetExceptionInformation()))
    {
        puts("in except");
    }
    puts("world");
}

Output

hello
in try
in try
in filter.
caught AV as expected.
in finally. termination:
        abnormal
in except
world

See also

Writing an exception handler
Structured Exception Handling (C/C++)
Keywords