In C++, std::string is a class that represents a dynamic string, i.e., a string of characters that can change in length during runtime. std::string objects store the characters in a contiguous block of memory, along with some additional metadata such as its length.

On the other hand, &str is a reference to a string slice. A string slice is a portion of a string that can be represented by a pointer to the first character of the slice and a length. Unlike std::string, string slices do not own the memory they point to, but rather borrow it from an underlying std::string or another string slice. This makes string slices much lighter-weight than std::string objects.

In short, if you need to store and manipulate a string of characters, use std::string. If you just need to refer to a portion of a string, use &str.
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