【C++17】std::optional

std::optional

1. vlaue_or 函数使用

#include <iostream>
#include <optional>
#include <string>

std::optional<std::string> create(bool b)
{
    if(b) return "Godzilla";
    return {};
}

int main()
{
    std::cout << "create(false) return: " << create(false).value_or("empty") << std::endl;
    std::cout << "create(true) return: " << create(true).value_or("empty") << std::endl;
    return 0;
}

输出：

create(false) return: empty
create(true) return: Godzilla

2. reset函数使用

void reset() noexcept;

 用例：

#include <iostream>
#include <optional>

struct Base
{
  std::string s;
  Base(std::string str) : s(std::move(str)) { std::cout << "constructed!" << std::endl; }
  ~Base() { std::cout << "destructed! " << std::endl; }

  Base(const Base &b) : s(b.s) { std::cout << "copy constructed!" << std::endl; }

  Base &operator=(const Base &rhs)
  {
    s = rhs.s;
    std::cout << " copy assigned!" << std::endl;
    return *this;
  }

  Base &operator=(Base &&rhs)
  {
    s = std::move(rhs.s);
    std::cout << " move assigned!" << std::endl;
    return *this;
  }
};

int main()
{
  std::cout << "Create empty optional: " << std::endl;
  std::optional<Base> opt;

  std::cout << "Construct and assign value: " << std::endl;
  opt = Base("Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.");

  std::cout << "Read option: " << std::endl;
  opt.reset();
  std::cout << "End example." << std::endl;
}

 输出：

Create empty option: 
Construct and assign value: 
constructed!
copy constructed!
destructed! 
Read option: 
destructed! 
End example!