Placement new input issue

[Drakonaut]

I'm stumped with a question. I wrote out an answer but it doesn't seem to be working. The inputs aren't the same as the outputs: it gives me some weird symbols for the name and 8 and 0 for the numbers.

#include <iostream>
#include <new>

struct chaff
{
    char dross[20];
    int slag;
};

int main()
{
    chaff stru1;
    chaff stru2;

    char stru1mem [sizeof(chaff)];
    char stru2mem [sizeof(chaff)];

    std::cout << "Enter the name for the 1st dude: ";
    std::cin.getline (stru1.dross, 20);
    std::cout << "Enter his number: ";
    std::cin >> stru1.slag;
    std::cin.ignore (100, '\n');
    std::cout << "Enter the name for the 2nd dude: ";
    std::cin.getline (stru2.dross, 20);
    std::cout << "Enter his number: ";
    std::cin >> stru2.slag;
    std::cin.ignore (100, '\n');

    chaff *stru1p = new (stru1mem) chaff;
    chaff *stru2p = new (stru2mem) chaff;

    std::cout << "1st dude's name: " << stru1p->dross << " and his number: " << stru1p->slag << std::endl;
    std::cout << "2nd dude's name: " << stru2p->dross << " and his number: " << stru2p->slag << std::endl;
}

Is there anything wrong with my workings?

Thanks!

[Peter87]

You have two separate things going on here.

1. You create two chaff objects stru1 and stru2 and read user input to assign values to the member variables.

chaff stru1;
chaff stru2;

std::cout << "Enter the name for the 1st dude: ";
std::cin.getline (stru1.dross, 20);
std::cout << "Enter his number: ";
std::cin >> stru1.slag;
std::cin.ignore (100, '\n');
std::cout << "Enter the name for the 2nd dude: ";
std::cin.getline (stru2.dross, 20);
std::cout << "Enter his number: ";
std::cin >> stru2.slag;
std::cin.ignore (100, '\n');

2. You create two chaff objects using placement new, but you never assign any values to the member variables so when you try to print them you get garbage.

char stru1mem [sizeof(chaff)];
char stru2mem [sizeof(chaff)];

chaff *stru1p = new (stru1mem) chaff;
chaff *stru2p = new (stru2mem) chaff;

std::cout << "1st dude's name: " << stru1p->dross << " and his number: " << stru1p->slag << std::endl;
std::cout << "2nd dude's name: " << stru2p->dross << " and his number: " << stru2p->slag << std::endl;

[Bdanielz]

I am assuming you were trying to use pointers to try and troubleshoot your issue.

#include <iostream>

struct chaff {
    char dross[20];
    int slag;
};

int main() {
    chaff stru1;
    chaff stru2;
    std::cout << "Enter the name for the 1st dude: ";
    std::cin.getline(stru1.dross, 20);
    std::cout << "Enter his number: ";
    std::cin >> stru1.slag;
    std::cin.ignore(100, '\n');
    std::cout << "Enter the name for the 2nd dude: ";
    std::cin.getline(stru2.dross, 20);
    std::cout << "Enter his number: ";
    std::cin >> stru2.slag;
    std::cin.ignore(100, '\n');
    std::cout << "1st dude's name: " << stru1.dross << " and his number: " << stru1.slag << std::endl;
    std::cout << "2nd dude's name: " << stru2.dross << " and his number: " << stru2.slag << std::endl;
}

[andywestken]

Is there anything wrong with my workings?

Using placement new with a old-fashioned struct is a bit pointless.

The mechanism is provided so you can invoke the constructor of your class independently of the memory allocation normally done by (regular, non-placement) new.

If you have no constructor there is zero point in using this approach.

Andy