What is the difference between String and &str in rust?
In Rust, String and &str are two different types that represent strings in different ways.

String is an owned, heap-allocated string type. This means that a String is a standalone string value that you can manipulate and control as you like. When you create a String, it will allocate memory on the heap to store its contents, and you are responsible for freeing this memory when you're done with the String.

&str is a string slice type. A string slice is a reference to a portion of a string, rather than a standalone value. This means that when you use a &str, you are not responsible for managing the memory associated with the string data; the data is stored elsewhere, and the &str is simply a reference to that data. This makes &str more lightweight and efficient than String, especially when you're only looking to access a small portion of a string.

In general, it is more efficient to use &str whenever you don't need to modify the string, as &str is a reference type and does not require heap allocation. On the other hand, you should use String if you need to modify the string data, as String is an owned type and provides methods for modifying its contents.
how about c++?
In C++, the equivalent of Rust's String is a std::string. This is a dynamically-allocated string type that is stored on the heap, and provides a number of methods for manipulating its contents. The equivalent of Rust's &str is a C-style string, or a char* in C++. This is a null-terminated string of characters stored in memory, and is used for string data that does not need to be modified. When working with C-style strings, it is important to keep in mind that they do not have automatic memory management like std::string, and it is up to the programmer to allocate and free the memory used by these strings.