Are Strings Created at Run-Time?

Can C-Strings or std::strings be created as constexpr or must they be created at run-time?

With gcc 4.9.2 I can do this:

constexpr const char foo[] = "blee";

(Sadly the November 2013 Customer Technology Preview does not allow Visual Studio to support this: http://ift.tt/19PeZWY)

But even with gcc 4.9.2 I cannot do this:

constexpr const std::string foo = "blee";

I get the error:

the type 'const string {aka const std::basic_string<char>}' of constexpr variable 'foo' is not literal

Does this mean that the conversion from "blee" to a std::string isn't defined until run-time, or that std::string can't be a literal, or what?