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Comment by Eiwatah4 | Hacker News Reader

What's interesting here to me is that C++ strings are not immutable. So I'd have expected them to behave basically the same way as StringBuilders in other languages. But apparently they are required to be stored continuously, and I guess that's what makes them slower here.

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Yes, I think the semantics of c_str() and data() effectively require that it is stored contiguously.

Although it is still possible to make it faster by overallocating in the same way as std::vector, but at the cost of more memory use.

I haven't looked at any implementation recently, but the standard specifically leaves open that implementatioms postpone joining string buffers until c_str() or data() is called (also, the pointers returned by those calls could contain copies of the strings; that is not something I would expect, but I see nothing in the standard that precludes it)

http://en.cppreference.com/w/cpp/string/basic_string/c_str

According to that link, c_str() and data() work in constant time. With that restriction, it's impossible to do the joining lazily - it must be done when data is added to the string.

Ha, it looks like they changed that in C++11. http://www.cplusplus.com/reference/string/string/data/ claims "Unspecified or contradictory specifications." for C++98, but constant complexity for C++11.

An answer to http://programmers.stackexchange.com/questions/124731/what-p... indicates that C++03 doesn't require constnat time, either.

Thanks for the education.

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