What’s Stack Unwinding?
Stack unwinding is the method of executing destructors for all native objects when an exception propagates out of a perform. It occurs when an exception is thrown and never caught throughout the identical perform. When this happens, the destructors for all objects with computerized storage length declared in that perform are known as in reverse order of their declaration earlier than the management is transferred to a handler (if any) or returned to the caller.
Stack unwinding is often clear to the programmer and occurs routinely. Stack Unwinding is often related to Exception Dealing with. When an exception happens in C++, the perform name stack is linearly looked for the exception handler, and any entries earlier than the perform with the exception handler are eradicated. If an exception just isn’t dealt with in the identical code, stack unwinding is required (the place it’s thrown). Stack unwinding is actually the method of invoking the destructors for all automated objects created at run time (each time an exception is thrown).
Alternative ways to method Stack Unwinding:
There are other ways of approaching the subject of stack unwinding in a destructor.
- A technique is to have a look at it from the angle of what occurs when an exception is thrown.
- One other method is to have a look at it from the angle of how a destructor is named when an object goes out of scope.
stack unwinding from the angle of an exception being thrown can assist us perceive why you will need to have a destructor that may clear up after itself.
When an exception is thrown:
This system execution jumps to the closest catch block. However earlier than that occurs, all of the objects that had been created within the strive block are destroyed. This consists of any native objects in addition to any objects that had been created by dynamic reminiscence allocation. If the destructors for these objects don’t correctly clear up, then it could result in reminiscence leaks or different issues.
From the angle of how a destructor is named:
It might probably assist to know the significance of a destructor. A destructor is named each time an object goes out of scope. When an object goes out of scope, its destructor is named and any assets that it was utilizing are freed up. If the destructor doesn’t correctly clear up, then it could result in useful resource leaks or different issues.
Tips on how to Detect Stack Unwinding?
In a destructor, stack unwinding might be detected by on the lookout for indicators of cleanup exercise, reminiscent of the next:
- Invoking capabilities that launch assets (e.g. shut recordsdata or free reminiscence)
- Logging messages
- Setting flags
If any of those actions are noticed in a destructor, it’s probably that stack unwinding is going down.
Utilizing the std::uncaught_exception()
You need to use the std::uncaught_exception() perform which returns true if an exception is presently being dealt with (i.e. thrown however not but caught). So, in your destructor, you’ll verify if std::uncaught_exception() returns true or false and take acceptable motion accordingly.
Overriding std::terminate() perform
In an effort to detect when stack unwinding is going on, you’ll be able to override the std::terminate() perform. The runtime calls this perform when it can not discover a appropriate exception handler. By overriding std::terminate(), you’ll be able to set a breakpoint or log a message to assist debug your program.
Right here is an instance of how you’ll override std::terminate():
By setting a flag within the constructor:
Set a flag within the constructor and verify that flag within the destructor. If the flag is ready, then you already know that the destructor was known as due to an exception. Right here is an instance: