What is the difference between passing const std::string& and std::string_view?

const std::string& avoids a copy but forces the caller to have (or construct) a std::string—a string literal creates a temporary std::string. std::string_view accepts any character source (literal, std::string, substring) without constructing a string, and is cheap to copy. Prefer string_view for read-only parameters, unless you need a null-terminated string or to store it.

void a(const std::string& s); // literal -> temp std::string
void b(std::string_view s);    // literal -> no allocation