Does C 11 Standard Prohibit Copy-on-Write Implementations for `std::string`?

Legality of Copy-on-Write (COW) Implementation in std::string of C 11

It has been a common belief that Copy-on-write (COW) is not a permissible way to realize a compliant implementation of std::string in C 11. However, recent discussions have challenged this notion.

Does C 11 Prohibit COW Implementations of std::string?

Yes, COW-based implementations of std::string are prohibited in C 11.

Standard Stipulations

The restriction is explicitly stated in the C 11 standard (section 21.4.1 p6):

"Invalidation of iterators/references is only allowed for:

• As an argument to any standard library function taking a reference to non-const basic_string as an argument.
• Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.*"

Implication for COW

For a COW string, calling the non-const operator[] would necessitate creating a copy (which would invalidate references). However, this action is prohibited by the standard, rendering COW implementations of std::string non-compliant in C 11.

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