Can we take a const ref of a substring?

Question

I have a class, which contains a string as a private member. The class has public member functions which all return a const ref to a std::wstring.

All of the const-ref strings being returned are substrings of the private member.

I could store each substring as members, then return const refs to those members, but this is not optimal as it duplicates memory.

Assume that I do store enough memory to know the start and end index of each substring.

How do I implement the above class without copying any of the member string?

Duplicate? [How can I pass a substring by reference?](https://stackoverflow.com/questions/15490246/how-can-i-pass-a-substring-by-reference). What is suggested here is passing an iterator, or treating it like a char*. — Nathan Cooper, Aug 06 '19 at 15:22
return `std::string_view` https://en.cppreference.com/w/cpp/string/basic_string_view instead? — Alan Birtles, Aug 06 '19 at 15:22
`string` or `wstring`? Are you using both, or is that a typo? — Marshall Clow, Aug 06 '19 at 15:40

NathanOliver · Accepted Answer · 2019-08-06T15:38:54.373

This is not legal, and it is not going to save you anything. std::string::substr returns by value. That means it makes a copy of the data from the class member no matter how you return it. When you return a reference to that, you are returning a reference to a temporary local object. the const reference lifetime extensions rules wont work here and what you are left with at the call site is a dangling reference.

What you can do though is just return by value. By default this will cause a move, and the compiler can quite easily optimize this away by eliding the temporary construction directly into the object in the call site.

Another option is to have a std::string_view member, and return sub strings from that. Doing that only costs you the copying of a pointer and a integer, so it will be quite fast.

Can we take a const ref of a substring?

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