Taking the address of a boost::any variable-CodePudding

Consider:

#include <boost/any.hpp>
#include <iostream>
#include <vector>
#include <string>

int main() {

    //Code snippet 1
    boost::any variable(std::string("Hello "));
    std::string* string_ptr_any = boost::any_cast<std::string>(&variable);
    std::cout << "Pointer value is " << string_ptr_any <<", variable's address is " << &variable << std::endl;
    //End Code snippet 1

    //Code snippet 2
    std::string normalstring = "hello_normal";
    std::string* normalstringptr = &normalstring;
    std::cout << "Pointer value is " << normalstringptr << ", variable's address is " << &normalstring << std::endl;
    //End Code snippet 2

    //Code snippet 3
    std::string& reference_of_string = boost::any_cast<std::string&>(variable);
    reference_of_string  = "World!";
    std::cout << reference_of_string << std::endl;
    std::cout << boost::any_cast<std::string>(variable) << std::endl;
    //End Code snippet 3    

    getchar();

}

Running this on compiler explorer gives as output:

Pointer value is 0xc2feb8, variable's address is 0x7ffec1620d68
Pointer value is 0x7ffec1620d40, variable's address is 0x7ffec1620d40
Hello World!
Hello World!

(Q1) As expected, Code snippet 2 gives the same value for both the address of the variable as well as the pointer (0x7ffec1620d40 in this case). But what explains the similar code in Code snippet 1 giving rise to two different addresses? (0xc2feb8 vs 0x7ffec1620d68) What is the relationship between these two addresses in memory?

(Q2) I am unable to see a consistent syntax rule to cast a boost::any variable. In Code snippet 1, to obtain a pointer, the syntax is:

std::string* string_ptr_any = boost::any_cast<std::string>(&variable);

while in Code snippet 3, to obtain a reference, the syntax is:

std::string& reference_of_string = boost::any_cast<std::string&>(variable);

More specifically, why is not the syntax in Code snippet 1:

std::string* string_ptr_any = boost::any_cast<std::string*>(&variable);

Is there a general rule of which these syntaxes are consistent special cases?

CodePudding user response：

But what explains the similar code in Code snippet 1 giving rise to two different addresses? (0xc2feb8 vs 0x7ffec1620d68) What is the relationship between these two addresses in memory?

string_ptr_any is the address of the std::string object. &variable is the address of the boost::any object.

why is not the syntax in Code snippet 1:
std::string* string_ptr_any = boost::any_cast<std::string*>(&variable);

Because the boost::any doesn't contain an object of type std::string*. You can only cast to the type that is contained, or a reference to it. This would be an alternative way to write snippet 1:

std::string* string_ptr_any = &boost::any_cast<std::string&>(variable);