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The variable declared by the exception-declaration, of type cv T or cv T&, is initialized from the exception object, of type E, as follows:
  — if T is a base class of E, the variable is copy-initialized from the corresponding base class subobject of the exception object;
  — otherwise, the variable is copy-initialized from the exception object.

Because the variable declared by the exception-declaration is copy-initialized, it is possible to slice the exception object as part of the copy operation, losing valuable exception information and leading to incorrect error recovery. For more information about object slicing, see OOP51-CPP. Do not slice derived objects. Further, if the copy constructor of the exception object throws an exception, the copy initialization of the exception-declaration object results in undefined behavior. (See ERR60-CPP. Exception objects must be nothrow copy constructible for more information.)

Always catch exceptions by lvalue reference unless the type is a trivial type. For reference, the C++ Standard, [basic.types], paragraph 9 [ISO/IEC 14882-2014], defines trivial types as the following:

Arithmetic types, enumeration types, pointer types, pointer to member types, std::nullptr_t, and cv-qualified versions of these types are collectively called scalar types.... Scalar types, trivial class types, arrays of such types and cv-qualified versions of these types are collectively called trivial types.

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Object slicing can result in abnormal program execution. This generally is not a problem for exceptions, but it can lead to unexpected behavior depending on the assumptions made by the exception handler.

Automated Detection

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