C++17之std::any的具体使用

std::any a; // a is empty
std::any b = 4.3; // b has value 4.3 of type double
 
a = 42; // a has value 42 of type int
b = std::string{"hi"}; // b has value "hi" of type std::string
 
if (a.type() == typeid(std::string)) 
{
    std::string s = std::any_cast<std::string>(a);
    useString(s);
}
else if (a.type() == typeid(int)) 
{
    useInt(std::any_cast<int>(a));
}

auto s = std::any_cast<std::string>(a);

try {
auto s = std::any_cast<std::string>(a);
...
}
catch (std::bad_any_cast& e) {
std::cerr << "EXCEPTioN: " << e.what() << '\n';
}

std::cout << std::any_cast<const std::string&>(a);

std::any_cast<std::string&>(a) = "world";

auto p = std::any_cast<std::string>(&a);
if (p) {
...
}

#include <iOStream>
#include <any>
 
int main()
{
    std::any i = 42;
    const auto ptr = std::any_cast<int>(&i);
    if (ptr)
    {
        std::cout << ptr << std::endl;
    }
 
    return 0;
}

if (a.has_value()) {
...
}

std::any a = "hello"; // type() is const char*
if (a.type() == typeid(const char*)) { // true
...
}
if (a.type() == typeid(std::string)) { // false
...
}
std::cout << std::any_cast<const char*>(v[1]) << '\n'; // OK
std::cout << std::any_cast<std::string>(v[1]) << '\n'; // EXCEPTION

#include <iostream>
#include <vector>
#include <any>
 
int main()
{
    std::vector<std::any> v;
    v.push_back(42);
    std::string s = "hello";
    v.push_back(s);
    for (const auto& a : v) {
        if (a.type() == typeid(std::string)) {
            std::cout << "string: " << std::any_cast<const std::string&>(a) << '\n';
        }
        else if (a.type() == typeid(int)) {
            std::cout << "int: " << std::any_cast<int>(a) << '\n';
        }
    }
}

namespace std {
class any;
}

std::any a1; // a1 is empty

std::any a2 = 42; // a2 contains value of type int
std::any a3 = "hello"; // a2 contains value of type const char*

std::any a4{std::in_place_type<long>, 42};
std::any a5{std::in_place_type<std::string>, "hello"};

std::any a5b{std::in_place_type<const char[6]>, "hello"};

std::any a6{std::complex{3.0, 4.0}};
std::any a7{std::in_place_type<std::complex<double>>, 3.0, 4.0};

// initialize a std::any with a set with lambda as sorting criterion:
auto sc = [] (int x, int y) { return std::abs(x) < std::abs(y);};
 
std::any a8{std::in_place_type<std::set<int,decltype(sc)>>, {4, 8, -7, -2, 0, 5}, sc};

auto a10 = std::make_any<float>(3.0);
auto a11 = std::make_any<std::string>("hello");
auto a13 = std::make_any<std::complex<double>>(3.0, 4.0);
auto a14 = std::make_any<std::set<int,decltype(sc)>>({4, 8, -7, -2, 0, 5}, sc);

std::any_cast<std::string>(a) // yield copy of the value
std::any_cast<std::string&>(a); // write value by reference
std::any_cast<const std::string&>(a); // read-access by reference

#include <iostream>
#include <string>
#include <any>
 
int main()
{
    const auto& s = std::make_any<std::string>("hello");
    
    if (s.type() == typeid(std::string))//删除顶层cosnt和引用后的类型
    {
        auto a = std::any_cast<std::string>(s);
        std::cout << a << std::endl;
    }
 
    return 0;
}

auto a = std::make_any<std::string>("hello");
std::any_cast<std::string>(&a) // write-access via pointer
std::any_cast<const std::string>(&a); // read-access via pointer

std::any_cast<std::string&>(&a); // RUN-TIME ERROR

#include <iostream>
#include <string>
#include <any>
#include <complex>
 
int main()
{
    std::any a;
    a = 42; // a contains value of type int
    a = "hello"; // a contains value of type const char*
 
    a.emplace<std::string>("hello world");// a contains value of type std::string
 
    return 0;
}

std::string s("hello, world!");
std::any a;
a = std::move(s); // move s into a
s = std::move(std::any_cast<string&>(a)); // move assign string in a to s

s = std::any_cast<string>(std::move(a));

std::any_cast<string&>(a) = std::move(s2); // OOPS: a to hold a string