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2475. Allow overwriting ofstd::basic_string terminator withcharT() to allow cleaner interoperation with legacy APIs

Section: 27.4.3.6[string.access]Status:C++17Submitter: Matt WeberOpened: 2015-02-21Last modified: 2017-07-30

Priority:3

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Discussion:

It is often desirable to use astd::basic_string object as a buffer when interoperating with libraries that mutate null-terminated arrays of characters. In many cases, these legacy APIs write a null terminator at the specified end of the provided buffer. Providing such a function with an appropriately-sizedstd::basic_string results in undefined behavior when thecharT object at thesize() position is overwritten, even if the value remains unchanged.

Absent the ability to allow for this, applications are forced into pessimizations such as: providing appropriately-sizedstd::vectors ofcharT for interoperating with the legacy API, and then copying thestd::vector to astd::basic_string; providing an oversizedstd::basic_string object and then callingresize() later.

A trivial example:

#include <string>#include <vector>void legacy_function(char *out, size_t count) {  for (size_t i = 0; i < count; ++i) {    *out++ = '0' + (i % 10);  }  *out = '\0'; //if size() == count, this results in undefined behavior}int main() {  std::string s(10, '\0');  legacy_function(&s[0], s.size()); //undefined behavior  std::vector<char> buffer(11);  legacy_function(&buffer[0], buffer.size() - 1);  std::string t(&buffer[0], buffer.size() - 1); // potentially expensive copy  std::string u(11, '\0');  legacy_function(&u[0], u.size() - 1);  u.resize(u.size() - 1); // needlessly complicates the program's logic}

A slight relaxation of the requirement on the returned object from the element access operator would allow for this interaction with no semantic change to existing programs.

[2016-08 Chicago]

Tues PM: This should also apply to non-constdata(). Billy to update wording.

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4296.

1. Edit 27.4.3.6[string.access] as indicated:

   const_reference operator[](size_type pos) const;reference operator[](size_type pos);

   -1-Requires: […]

   -2-Returns:*(begin() + pos) ifpos < size(). Otherwise, returns a reference to an object of typecharT with valuecharT(), where modifying the objectto any value other thancharT() leads to undefined behavior.

   […]