[diff.cpp20.strings]

# Annex C (informative) Compatibility [[diff]](./#diff)

## C.2 C++ and ISO C++ 2020 [[diff.cpp20]](diff.cpp20#strings)

### C.2.11 [[strings]](strings "27 Strings library"): strings library [diff.cpp20.strings]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L749)

**Affected subclause:** [[string.classes]](string.classes)  


**Change:** Additional rvalue overload for the substr member function and
the corresponding constructor[.](#1.sentence-1)

  


**Rationale:** Improve efficiency of operations on rvalues[.](#1.sentence-2)

  


**Effect on original feature:** Valid C++ 2020 code that created a substring
by calling substr (or the corresponding constructor)
on an xvalue expression with type S that is a specialization of basic_string may change meaning in this revision of C++[.](#1.sentence-3)

[*Example [1](#example-1)*: std::string s1 = "some long string that forces allocation", s2 = s1;
std::move(s1).substr(10, 5);
assert(s1 == s2); // unspecified, previously guaranteed to be true std::string s3(std::move(s2), 10, 5);
assert(s1 == s2); // unspecified, previously guaranteed to be true — *end example*]