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any_cast doc: https://en.cppreference.com/w/cpp/utility/any/any_cast

This blog explains the std::any_cast behavior. The std::any_cast is a template function that casts std::any to a reference to the type T stored in the std::any. The std::any_cast function is used to retrieve the value stored in the std::any object. The std::any_cast function is a non-throwing function. If the std::any object does not contain a value of type T, the function returns a null pointer.

Some examples are shown below:

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#include <iostream>
#include <string>
#include <any>

using namespace std;

int main() {
  // explain the difference between auto& and auto
  auto a1 = any(12);  //necessary
  a1 = string("test");
  auto& s = any_cast<string&>(a1);
  s[0] = "s";
  cout << any_cast<string const&>(a1) << endl;   // output sest
  return 0;
}
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#include <iostream>
#include <string>
#include <any>

using namespace std;

int main() {
  auto a1 = any(12);  //necessary
  a1 = string("test");
  auto s = any_cast<string>(a1);
  s[0] = "s";
  cout << any_cast<string const&>(a1) << endl;   // output test
  return 0;
}
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#include <iostream>
#include <string>
#include <any>
#include <cassert>
#include <unordered_map>
#include <memory>
using namespace std;

void test1() {
        auto a1 = any(12);
        a1 = string("test");
        auto& ra = any_cast<string&>(a1);
        ra[0] = 's';
        assert(any_cast<string const&>(a1) == "sest");
        return ;

}

void test10() {
        auto a1 = any(12);
        a1 = string("test");
        auto ra = any_cast<string>(a1);  // cannot use auto& if any_cast<string>, will compliation error
        ra[0] = 's';
        assert(any_cast<string const&>(a1) == "test");
        return ;

}

void test2() {
        unordered_map<string, any> m;
        m["s"] = string("test");
        auto& ra = any_cast<string&>(m["s"]);
        ra[0] = 's';
        assert(any_cast<string const&>(m["s"]) == "sest");
        //cout << s << endl;
        return ;
}

int main() {
        test1();
        test10();
        test2();
        return 0;
}