std::exception requires that its constructor be throw(). Yet std::runtime_error accepts a std::string as its argument, which indicates that it's storing a std::string somewhere. Therefore, an assignment or copy construction has to be going on somewhere. And for std::string, that's not a nothrow operation.
How then does runtime_error::runtime_error meet throw()?
(For context, I'm implementing an exception type, and want to store a few std::strings from the call site, and I want to do it correctly...)
(Here's the same thing in a minimal-ish testcase.)
runtime_error::runtime_error(string const&) doesn't need to meet throw().
It doesn't inherit from or override exception::exception(), and by the time string's copy constructor is invoked, exception::exception() has completed.
If copying the string were to throw an exception, this would unwind runtime_error::runtime_error(string const&) and then, I suppose, invoke exception::~exception().
It's hard to directly show that there is no requirement of a derived ctor to meet a base ctor's exception specifier, but it is strongly implied by the following passage (which describes how the base's destructor is invoked, rather than passing the exception into the base constructor):
[2003: 15.2/2]An object that is partially constructed or partially destroyed will have destructors executed for all of its fully constructed subobjects, that is, for subobjects for which the constructor has completed execution and the destructor has not yet begun execution. Should a constructor for an element of an automatic array throw an exception, only the constructed elements of that array will be destroyed. If the object or array was allocated in a new-expression, the matching deallocation function (3.7.3.2, 5.3.4, 12.5), if any, is called to free the storage occupied by the object.
The only passage which comes even close to the scenario you presumed (and which I initially presumed) is the following.
[2003: 15.4/3]If a virtual function has an exception-specification, all declarations, including the definition, of any function that overrides that virtual function in any derived class shall only allow exceptions that are allowed by the exception-specification of the base class virtual function.
But clearly exception::exception() is not a virtual function, and clearly runtime_error::runtime_error(string const&) does not override it.
(Note that this scenario would apply for a virtual destructor; accordingly, you can see that, in libstdc++, runtime_error::~runtime_error() is throw()).