tmp/tmpivczlbnb/{from.md → to.md}
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### [[strings]]: strings library <a id="diff.cpp20.strings">[[diff.cpp20.strings]]</a>
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**Change:** Additional rvalue overload for the `substr` member function
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and the corresponding constructor. **Rationale:** Improve efficiency of
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operations on rvalues. **Effect on original feature:** Valid C++20 code
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that created a substring by calling `substr` (or the corresponding
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constructor) on an xvalue expression with type `S` that is a
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specialization of `basic_string` may change meaning in this revision of
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C++. For example:
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``` cpp
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std::string s1 = "some long string that forces allocation", s2 = s1;
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std::move(s1).substr(10, 5);
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assert(s1 == s2); // unspecified, previously guaranteed to be true
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std::string s3(std::move(s2), 10, 5);
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assert(s1 == s2); // unspecified, previously guaranteed to be true
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```
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