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C++17 in Detail: A Deep Dive

Object Creation

Explore various methods to create std::any objects in C++17 including default initialization, direct initialization, and in-place construction. Understand how to change stored values safely and manage object lifetimes to prevent resource leaks.

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There are several ways you can create std::any object:

  • a default initialisation - then the object is empty
  • a direct initialisation with a value/object
  • in place std::in_place_type
  • via std::make_any

You can see it in the following example:

C++ 17
#include <string>
#include <iostream>
#include <any>
#include <cassert>
using namespace std;
class MyType
{
  int a, b;
  
  public:
    MyType(int x, int y) : a(x), b(y) { }
};
int main()
{
    // default initialization:
    std::any a;
    assert(!a.has_value());
    // initialization with an object:
    std::any a2{10}; // int
    std::cout <<  "a2 is: " << std::any_cast<int>(a2) << '\n';
    std::any a3{MyType{10, 11}};
    // in_place:
    std::any a4{std::in_place_type<MyType>, 10, 11};
    std::any a5{std::in_place_type<std::string>, "Hello World"};
    std::cout <<  "a5 is: " << std::any_cast<std::string>(a5) << '\n';
    // make_any
    std::any a6 = std::make_any<std::string>("Hello World");
    std::cout <<  "a6 is: " << std::any_cast<std::string>(a6) << '\n';
}

In Place Construction

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