| Issue |
56031
|
| Summary |
[libc+] std::basic_string::insert pessimization for custom character types
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| Labels |
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| Assignees |
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| Reporter |
treh
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std::basic_string::insert unnecessarily always copies into a `__temp` buffer if `charT` is a custom type.
[The implementation](https://github.com/llvm/llvm-project/blob/c36870c8e79c1690432076c69cb98a879555efa8/libcxx/include/string#L2930-L2949) uses [`__string_is_trivial_iterator`](https://github.com/llvm/llvm-project/blob/c36870c8e79c1690432076c69cb98a879555efa8/libcxx/include/string#L604-L613) to determine if it should copy the input range into a `basic_string __temp` buffer, before calling `__insert_from_safe_copy`.
However, for custom class-type `_CharT`, even a plain `_CharT const*` does not satisfy `__string_is_trivial_iterator`, so nothing is won by creating this buffer.
Shouldn't the definition for `__string_is_trivial_iterator` use something like `is_nothrow_convertible<Tp, _CharT>` instead of `is_arithmetic<Tp>`?
Note: Custom class-type character types **are** allowed in `basic_string`, the [requirements are](https://timsong-cpp.github.io/cppwp/n4861/strings#basic.string-1)
> The class template basic_string describes objects that can store a sequence consisting of a varying number of arbitrary char-like objects with the first element of the sequence at position zero. Such a sequence is also called a “string” if the type of the char-like objects that it holds is clear from context. In the rest of this Clause, the **type of the char-like objects** held in a basic_string object is designated by **charT**.
and *char-like* is defined [before](https://timsong-cpp.github.io/cppwp/n4861/strings#general-1) as
> This Clause describes components for manipulating sequences of **any non-array trivial standard-layout type. Such types are called char-like types**, and objects of char-like types are called char-like objects or simply characters.
Note2: This even was a bug from [this]( https://reviews.llvm.org/D98573) to [this](https://reviews.llvm.org/D119633) revision, causing infinite recursion.
Here is a small example to play around with: [main.cpp](https://github.com/llvm/llvm-project/files/8899669/main.cpp.txt)
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