【C++17】std::optional

std::optional

1. vlaue_or 函数使用

 1 #include <iostream>
 2 #include <optional>
 3 #include <string>
 4 
 5 std::optional<std::string> create(bool b)
 6 {
 7     if(b) return "Godzilla";
 8     return {};
 9 }
10 
11 int main()
12 {
13     std::cout << "create(false) return: " << create(false).value_or("empty") << std::endl;
14     std::cout << "create(true) return: " << create(true).value_or("empty") << std::endl;
15     return 0;
16 }

输出：

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

2. reset函数使用

void reset() noexcept;

用例：

 1 #include <iostream>
 2 #include <optional>
 3 
 4 struct Base
 5 {
 6   std::string s;
 7   Base(std::string str) : s(std::move(str)) { std::cout << "constructed!" << std::endl; }
 8   ~Base() { std::cout << "destructed! " << std::endl; }
 9 
10   Base(const Base &b) : s(b.s) { std::cout << "copy constructed!" << std::endl; }
11 
12   Base &operator=(const Base &rhs)
13   {
14     s = rhs.s;
15     std::cout << " copy assigned!" << std::endl;
16     return *this;
17   }
18 
19   Base &operator=(Base &&rhs)
20   {
21     s = std::move(rhs.s);
22     std::cout << " move assigned!" << std::endl;
23     return *this;
24   }
25 };
26 
27 int main()
28 {
29   std::cout << "Create empty optional: " << std::endl;
30   std::optional<Base> opt;
31 
32   std::cout << "Construct and assign value: " << std::endl;
33   opt = Base("Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.");
34 
35   std::cout << "Read option: " << std::endl;
36   opt.reset();
37   std::cout << "End example." << std::endl;
38 }

输出：

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