move constructor for std::runtime_error

Question

Why does std::runtime_error not provide a constructor accepting an std::string&&? Looking at the constructors for std::string, it has a move co

Answer 1

explicit runtime_error(string&&);

does not exist simply because it would not provide any optimization.

As it turns out, a C++11-conforming runtime_error does not internally store a std::string. The reason is that the copy members of runtime_error must not throw exceptions. Otherwise the wrong exception could get thrown when the compiler copies the exception object in the process of throwing it.

This implies that runtime_error needs to store a non-mutable reference counted string. However C++11 outlaws the COW-implementation for std::string. Implementations of std::string have moved to a "short-string-optimization" which must allocate on copy construction if the length of the string is beyond the "short limit". And there is no limit on the length of strings used to construct a runtime_error.

So effectively C++11 (and forward) contains two implementations of strings:

1. std::string : This is typically a short-string-optimized type with a copy constructor and copy assignment that is capable of throwing exceptions.

2. std::runtime_error : This is (or holds) an immutable reference-counted string. This will never throw on copy construction or copy assignment.

And

explicit runtime_error(string&&);

can never (efficiently) transfer resources from the "type 1" string to the "type 2" string.