C++ std::string Using the std::string_view class
Example
C++17
C++17 introduces std::string_view, which is simply a non-owning range of const chars, implementable as either a pair of pointers or a pointer and a length. It is a superior parameter type for functions that requires non-modifiable string data. Before C++17, there were three options for this:
void foo(std::string const& s); // pre-C++17, single argument, could incur
// allocation if caller's data was not in a string
// (e.g. string literal or vector<char> )
void foo(const char* s, size_t len); // pre-C++17, two arguments, have to pass them
// both everywhere
void foo(const char* s); // pre-C++17, single argument, but need to call
// strlen()
template <class StringT>
void foo(StringT const& s); // pre-C++17, caller can pass arbitrary char data
// provider, but now foo() has to live in a header
All of these can be replaced with:
void foo(std::string_view s); // post-C++17, single argument, tighter coupling
// zero copies regardless of how caller is storing
// the data
Note that std::string_view cannot modify its underlying data.
string_view is useful when you want to avoid unnecessary copies.
It offers a useful subset of the functionality that std::string does, although some of the functions behave differently:
std::string str = "lllloooonnnngggg sssstttrrriiinnnggg"; //A really long string
//Bad way - 'string::substr' returns a new string (expensive if the string is long)
std::cout << str.substr(15, 10) << '\n';
//Good way - No copies are created!
std::string_view view = str;
// string_view::substr returns a new string_view
std::cout << view.substr(15, 10) << '\n';
