Why is value taking setter member functions not recommended in Herb Sutter's CppCon 2014 talk (Back to Basics: Modern C++ Style)?

Question

In Herb Sutter\'s CppCon 2014 talk Back to Basics: Modern C++ Style he refers on slide 28 (a web copy of the slides are here) to this pattern:

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Answer 1

You have two ways of calling that methods.

• With rvalue parameter, as long as the move constructor of the parameter type is noexcept there is no problem (in case of std::string most probably is noexcept), in any case would be better use a conditional noexcept (to be sure the parameters is noexcept)
• With lvalue parameter, in this case the copy constructor of the parameter type would be called and almost sure it will need some allocation (that could throw).

In cases like this that the use could be miss used, it's better to avoid. The client of the class assume that no exception is throw as specified, but in valid, compilable, not suspicious C++11 could throw.