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When an exception is thrown, the value of the object in the throw expression is used to initialize an anonymous temporary object called the exception object. The type of this exception object is used to transfer control to the nearest catch handler which contains an exception declaration with a matching type. The C++ Standard, [except.handle], paragraph 16 [ISO/IEC 14882-2014], states, in part:

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.

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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

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Bibliography

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