Inside the Microsoft STL: The std::exception_ptr

Raymond Chen

Raymond

When debugging, you may find yourself staring at a std::exception_ptr and want to know what exception is inside it.

What you see in the MSVC header file is that a std::exception_ptr is a class that consists of two pointers enigmatically named _Data1 and _Data2.

The dirty secret is that a std::exception_ptr is a std::shared_ptr in disguise.

Prerequisite: Advanced STL, part 1: shared_ptr by Stephan T. Lavavej.

The _Data1 acts as the _Ptr and points to the shared object. The _Data2 acts as the _Rep and points to the control block.

For debugging purposes, you can ignore the _Data2 and focus on the _Data1, which is a pointer to an EXCEPTION_RECORD.

Once you have the EXCEPTION_RECORD, you can use the .exr command to view it, and then use the existing cookbook for extracting the thrown object and its type information.

In practice, you don’t usually need to go through the whole cookbook. The Parameter[1] points to the object that was thrown, and that object usually contains enough information to let you figure out what it is.

We’ll look at some of the possibilities next time.

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