Is Copy-on-Write Implementation of std::string Legal in C 11?

Legality of COW std::string Implementation in C 11

In earlier versions of C , copy-on-write (COW) was a viable implementation strategy for std::string. However, concerns have been raised about its validity in C 11. This article examines the legality of COW-based std::string implementations in C 11.

Question: Does C 11 explicitly prohibit COW-based std::string implementations?

Answer: No, C 11 does not explicitly state that COW-based implementations are prohibited.

Question: If not, how is this restriction implied?

Answer: The restriction arises from the new requirements in C 11 regarding iterator and reference invalidation. According to section 21.4.1 p6 of the standard, invalidation is only permitted in specific cases:

• Passing as an argument to specific standard library functions that accept non-const basic_string references.
• Calling non-const member functions, excluding operator[], at, front, back, begin, rbegin, end, and rend.

In a COW string implementation, calling non-const operator[] would necessitate a copy and reference invalidation. However, the aforementioned paragraph prohibits this. Therefore, COW std::string implementations are no longer permissible in C 11.

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