Strange pointer aliasing problem

[gbpic]

I'm learning about pointers and structs, so I wrote a little demo program. I'm compiling my code with the GNU G++ compiler.

I have a struct that looks like this:

    struct Robot{
        string name;
        int arms;
        int lasers;
        int nukes;
        int heart;

        void haveAHeart(){
            nukes=0;
            lasers=0;
            heart=1;
            cout << "\t" << name << ": 'Humans aren't so bad...'\n";
        }

        void killEverything(){
            nukes=0;
            cout << "\t" << name << ": 'Would you like to play a game?' *boom*\n";
        }
        
        void setName(string name){
            this->name = name;
        }
        
        string getName(){
            return name;
        }
    };

And I have some code like this:

    //now things are going to get nuts up in here
    Robot * robots[5];
    
    robots[0] = &robbie; //load in robbie's address
    cout << "Loaded " << robots[0]->name << " into the array\n";
    for(int i=1; i<5; i++){ //make generic robots
        Robot r; //despite this, every pointer will point to the same Robot r
        r.setName("Gamma");
        r.arms=2;
        r.lasers=2;
        r.nukes=0;
        r.heart=-1;
        robots[i] = &r;
    }
    
    Robot s; //let's make a unique robot (note that the variable name is different)
    s.setName("Sigma");
    s.arms = 4;
    s.lasers = 17;
    s.nukes = 5;
    s.heart = -2;
    robots[1] = &s; //store it in the list
    
    /*
    Robot b;   //new robot
    Robot * bPtr = &b;   //make a pointer for the new robot
    *(Robot *)bPtr.name = "Bob";   //this does not compile

    bPtr->name = "Bob";   //use this instead
    */
    
    cout << "We made a bunch of generic Robots\n"
         << "Here is a list of all the robots:\n";
    for(int i=0; i<5; i++){
        cout << "\t" << robots[i]->name << " (pointer:" << robots[i] << ")\n"
             << "\t\tarms: " << robots[i]->arms << "\n"
             << "\t\tlasers: " << robots[i]->lasers << "\n"
             << "\t\tnukes: " << robots[i]->nukes << "\n"
             << "\t\theart: " << robots[i]->heart << "\n";
    }

Note: robbie is an existing Robot struct I had.

My problem is, when I print out a list of the names of the robots, I expect Robbie, Sigma, Gamma, Gamma, Gamma. Instead, I'm getting Robbie, Sigma, Sigma, Sigma, Sigma. Here's my output:

Here is a list of all the robots:
	Robbie (pointer:0x7fff5fbff920)
		arms: 2
		lasers: 0
		nukes: 0
		heart: 1
	Sigma (pointer:0x7fff5fbff8e0)
		arms: 4
		lasers: 17
		nukes: 5
		heart: -2
	Sigma (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1
	Sigma (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1
	Sigma (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1

The pointer for the first Sigma is different than all the other ones, which baffles me even more.

[gbpic]

Additionally, if I change r.setName("Gamma") to r.setName("sadfjdksalfjlsd") it works just fine:

Here is a list of all the robots:
	Robbie (pointer:0x7fff5fbff920)
		arms: 2
		lasers: 0
		nukes: 0
		heart: 1
	Sigma (pointer:0x7fff5fbff8e0)
		arms: 4
		lasers: 17
		nukes: 5
		heart: -2
	sadfjdksalfjlsd (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1
	sadfjdksalfjlsd (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1
	sadfjdksalfjlsd (pointer:0x7fff5fbff900)
		arms: 2
		lasers: 2
		nukes: 0
		heart: -1

I've compiled and run it on both Linux and MacOS and I get the same behavior. It's really weird.

[kbw]

    Robot * robots[5];
    //...
    for(int i=1; i<5; i++){ //make generic robots
        Robot r; //despite this, every pointer will point to the same Robot r
        r.setName("Gamma");
        r.arms=2;
        r.lasers=2;
        r.nukes=0;
        r.heart=-1;
        robots[i] = &r;
    }

This creates a Robot object r, assigns the address to robots[i], then destroys the r.

You probably need to allocate them from the heap, but as it stands you'll end up with robots[0] not being from the heap, and the other from the heap. Your delete code will need to know this. It's better to allocate all, including Robbie, from the heap.

[gbpic]

Thanks for the help. For future reference, I fixed it by changing the for loop to:

    for(int i=1; i<5; i++){ //make generic robots
        Robot * r;
        r = new Robot;
        r->setName("Gamma");
        r->arms=2;
        r->lasers=2;
        r->nukes=0;
        r->heart=-1;
        robots[i] = r;
    }

Is this what you were referring to when you said to "allocate them from the heap"? (I did notice that their addresses are now much lower and unique)

[kbw]

Yes.

[gbpic]

Thank you very much.