try-except Statement

 

The latest version of this topic can be found at try-except Statement.

Microsoft Specific**

The following syntax describes a try-except statement:

Syntax

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

Remarks

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

For related information, see the try-finally statement.

Exceptions can be either hardware-based or software-based. Even when applications cannot completely recover from hardware or software exceptions, structured exception handling makes it possible to display error information and trap the internal state of the application to help diagnose the problem. This is especially useful for intermittent problems that cannot be reproduced easily.

Note

Structured exception handling works with Win32 for both C and C++ source files. However, it is 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, it is recommended that you use the C++ exception-handling mechanism (try, catch, and throw statements).

The compound statement after the __try clause is the body or guarded section. The compound statement after the __except clause is the exception handler. The handler specifies a set of actions to be taken if an exception is raised during execution of the body of the guarded 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 (called the filter expression) is evaluated and the value determines how the exception is handled. There are three values:

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

   EXCEPTION_CONTINUE_SEARCH (0) Exception is not 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.

Because the __except expression is evaluated as a C expression, it is 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 is not valid to jump into a __try statement, but valid to jump out of one. The exception handler is not called if a process is terminated in the middle of executing a try-except statement.

For more information, see Knowledge Base article Q315937 : HOW TO: Trap Stack Overflow in a Visual C++ Application.

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 does not degrade performance as it does in a try-finally statement because stack unwinding does not occur. However, we recommend that you use the __leave keyword rather than a goto statement because you are 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 a 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:

struct _EXCEPTION_POINTERS {  
      EXCEPTION_RECORD *ExceptionRecord,  
      CONTEXT *ContextRecord }  

The pointer types _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 is 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  

END Microsoft Specific

See Also

Writing an Exception Handler
Structured Exception Handling (C/C++)
Keywords