C++17 使用 std::string_view避免字符串拷贝优化程序性能

void fun(const std::string& s) {
  std::cout << s << std::endl;
}

const char* ch = "hello world";

// bad way, expensive if the string is long
fun(ch); 

// very long string
std::string str = "lllloooonnnngggg sssstttrrriiinnnggg"; 

// bad way, expensive if the string is long
std::cout << str.substr(15, 10) << '\n';

constexpr basic_string_view() noexcept; (since C++17)
constexpr basic_string_view(const basic_string_view& other) noexcept = default; (since C++17)
constexpr basic_string_view(const CharT* s, size_type count)；(since C++17)
constexpr basic_string_view(const CharT* s); (since C++17)
template<class It, class End>

std::string str("hello string view!");
std::string_view sview(str);

operator basic_string_view<charT, traits>() const noexcept;

void fun(const std::string_view& s) {
  std::cout << s << std::endl;
}

const char* ch = "hello world, char";
fun(ch); 

const std::string str = "hello world, string";
fun(str);

fun({ch, 5});

// very long string
std::string str = "lllloooonnnngggg sssstttrrriiinnnggg";

//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';

std::string_view sv = std::string("hello world");

std::string_view fun() {
 std::string str("hello world");
 return std::string_view(str);
}

#include <iostream>
#include <vector>
#include <string>
#include <string_view>

int main() {
  const char* ch = "hello world";
  
  std::string_view sv(ch, 2);
  
  std::cout << sv << std::endl;
  
  std::cout << sv.data() << std::endl;
  
  return 0;
}

he
hello world