/EH (Exception Handling Model)

Article
02/20/2018

The latest version of this topic can be found at -EH (Exception Handling Model).

Specifies the kind of exception handling used by the compiler, when to optimize away exception checks, and whether to destroy C++ objects that go out of scope because of an exception. If /EH is not specified, the compiler catches both asynchronous structured exceptions and C++ exceptions, but does not destroy C++ objects that go out of scope because of an asynchronous exception.

Syntax

/EH{s|a}[c][r][-]

Arguments

a
The exception-handling model that catches both asynchronous (structured) and synchronous (C++) exceptions.

s
The exception-handling model that catches C++ exceptions only and tells the compiler to assume that functions declared as extern "C" may throw an exception.

c
If used with s (/EHsc), catches C++ exceptions only and tells the compiler to assume that functions declared as extern "C" never throw a C++ exception.

/EHca is equivalent to /EHa.

r
Tells the compiler to always generate runtime termination checks for all noexcept functions. By default, runtime checks for noexcept may be optimized away if the compiler determines the function calls only non-throwing functions.

Remarks

The /EHa compiler option is used to support asynchronous structured exception handling (SEH) with the native C++ catch(...) clause. To implement SEH without specifying /EHa, you may use the __try, __except, and __finally syntax. Although Windows and Visual C++ support SEH, we strongly recommend that you use ISO-standard C++ exception handling (/EHs or /EHsc) because it makes code more portable and flexible. Nevertheless, in existing code or for particular kinds of programs—for example, in code compiled to support the common language runtime (/clr (Common Language Runtime Compilation))—you still might have to use SEH. For more information, see Structured Exception Handling (C/C++).

Specifying /EHa and trying to handle all exceptions by using catch(...) can be dangerous. In most cases, asynchronous exceptions are unrecoverable and should be considered fatal. Catching them and proceeding can cause process corruption and lead to bugs that are hard to find and fix.

If you use /EHs or /EHsc, then your catch(...) clause does not catch asynchronous structured exceptions. Access violations and managed System.Exception exceptions are not caught, and objects in scope when an asynchronous exception is generated are not destroyed even if the asynchronous exception is handled.

If you use /EHa, the image may be larger and might perform less well because the compiler does not optimize a try block as aggressively. It also leaves in exception filters that automatically call the destructors of all local objects even if the compiler does not see any code that can throw a C++ exception. This enables safe stack unwinding for asynchronous exceptions as well as for C++ exceptions. When you use /EHs, the compiler assumes that exceptions can only occur at a throw statement or at a function call. This allows the compiler to eliminate code for tracking the lifetime of many unwindable objects, and this can significantly reduce code size.

We recommend that you not link objects compiled by using /EHa together with objects compiled by using /EHs in the same executable module. If you have to handle an asynchronous exception by using /EHa anywhere in your module, use /EHa to compile all the code in the module. You can use structured exception handling syntax in the same module as code that's compiled by using /EHs, but you can’t mix the SEH syntax with try, throw, and catch in the same function.

Use /EHa if you want to catch an exception that's raised by something other than a throw. This example generates and catches a structured exception:

  
      // compiler_options_EHA.cpp  
// compile with: /EHa  
#include <iostream>  
#include <excpt.h>  
using namespace std;  
  
void fail() {   // generates SE and attempts to catch it using catch(...)  
   try {  
      int i = 0, j = 1;  
      j /= i;   // This will throw a SE (divide by zero).  
      printf("%d", j);   
   }  
   catch(...) {   // catch block will only be executed under /EHa  
      cout<<"Caught an exception in catch(...)."<<endl;  
   }  
}  
  
int main() {  
   __try {  
      fail();   
   }  
  
   // __except will only catch an exception here  
   __except(EXCEPTION_EXECUTE_HANDLER) {     
   // if the exception was not caught by the catch(...) inside fail()  
      cout << "An exception was caught in __except." << endl;  
   }  
}

The /EHc option requires that /EHs or /EHa is specified. The /clr option implies /EHa (that is, /clr /EHa is redundant). The compiler generates an error if /EHs[c] is used after /clr. Optimizations do not affect this behavior. When an exception is caught, the compiler invokes the class destructor or destructors for the object or objects that are in the same scope as the exception. When an exception is not caught, those destructors are not run.

For information about exception handling restrictions under /clr, see _set_se_translator.

The option can be cleared by using the symbol -. For example, /EHsc- is interpreted as /EHs /EHc- and is equivalent to /EHs.

The /EHr compiler option forces runtime termination checks in all functions that have a noexcept attribute. By default, runtime checks may be optimized away if the compiler back end determines that a function only calls non-throwing functions. Non-throwing functions are any functions that have an attribute that specifies no exceptions may be thrown. This includes functions marked noexcept, throw(), __declspec(nothrow), and, when /EHc is specified, extern "C" functions. Non-throwing functions also include any that the compiler has determined are non-throwing by inspection. You can explicitly set the default by using /EHr-.

However, the non-throwing attribute is not a guarantee that no exceptions can be thrown by a function. Unlike the behavior of a noexcept function, the Visual C++ compiler considers an exception thrown by a function declared using throw(), __declspec(nothrow), or extern "C" as undefined behavior. Functions that use these three declaration attributes do not enforce runtime termination checks for exceptions. You can use the /EHr option to help you identify this undefined behavior, by forcing the compiler to generate runtime checks for unhandled exceptions that escape a noexcept function.

To set this compiler option in the Visual Studio development environment

Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.
In the left pane, expand Configuration Properties, C/C++, Code Generation.
Modify the Enable C++ Exceptions property.

Or, set Enable C++ Exceptions to No, and then on the Command Line property page, in the Additional Options box, add the compiler option.

To set this compiler option programmatically

See ExceptionHandling.

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