Question

Recently I have been working on sockets in C++ and I have come across this:

*(struct in_addr*)&serv_addr.sin_addr.s_addr = *(struct in_addr *)server->h_addr;

While this does do what I want it to I am a little confused as to why I can’t do this:

(struct in_addr)serv_addr.sin_addr.s_addr = *(struct in_addr *)server->h_addr;

Since it becomes a pointer and then immediately is dereferenced shouldn’t the second work as well as the first? I am still new to C++ and this is a little confusing to me. Any help would be greatly appreciated. Below is the code. All it does is takes the host name or IP and prints the IP to the screen.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>


using namespace std;

int main(){

    int socketfd, portno, rwResult; 
    struct sockaddr_in serv_addr; 
    struct hostent* server;     
    char inputName[50];

    //The next block gets the host name and port number and stores them in variables
    cout<<"Enter host(Max Size 50): ";
    cin>>inputName;
    cout<<endl<<"Enter port number: ";
    cin>>portno;
    cout<<endl;

    server = gethostbyname(inputName);
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(portno);

    *(struct in_addr*)&serv_addr.sin_addr.s_addr = *(struct in_addr *)server->h_addr;
    //This is where I am confused
    //(struct in_addr)serv_addr.sin_addr.s_addr = *(struct in_addr *)server->h_addr;


    cout<< "Server: "<<inet_ntoa(*(struct in_addr *)server->h_addr_list[0])<<endl;

    cout<< "Server: "<<inet_ntoa(*(struct in_addr *)&serv_addr.sin_addr.s_addr)<<endl;
    //The two cout's tell me if the address was copied correctly to serv_addr I believe.



    return 0;
}

Answer 1

A simpler example might help to explain the difference:

double q = 0.5;
int n;

n = (int) q;    // #1
n = *(int*)(&q) // #2

The first version converts the value of q into a value of type int according to the rules of the language. Conversion is only possible for primitive types and for classes that define conversion operators/constructors.

The second version reinterprets the binary representation of q as an integer. In good cases (like in your example) this produces something useful, but in general this is undefined behaviour, as it is not allowed to access a variable of one type through a pointer to a different type.

Your example may be valid because the two types in question are both POD structs with the same initial members, and you are accessing only one of the common initial members. Edit. I checked, server->h_addr is of type char *. It’s possible that this just serves as a placeholder and the pointer is in fact to a structure of the correct type.