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Can't figure out this subtle error

This is a discussion on Can't figure out this subtle error within the C++ Programming forums, part of the General Programming Boards category; Code: struct node{ string location; int gval; int fval; node *next; }; Code: struct node *hp; for(int i = 0; ...

  1. #1
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    Can't figure out this subtle error

    Code:
    struct node{
        string location;
        int gval;
        int fval;
        node *next;
    };
    Code:
            struct node *hp;
            for(int i = 0; i<graph.size();i++)
    	{
    		cout << graph[i].location <<"=="<<graph[0].location<<"?\n";
    		if((graph[i].location).compare(start)==0)
    		{ 
    			index = i;
    			break;
    		}
    	}
        cout<<"Found "<< graph[index].location;
        //make starting nodes
        hp = (struct node*)(malloc(sizeof(struct node)));
        hp->location = graph[index].location;
    For some reason the cout produces output fine with the string value of graph[index].location but when I debug my program, it crashes right at
    Code:
     hp->location = graph[index].location;
    Also graph is defined as
    Code:
    vector<vertice> graph = initializeGraph();
    Code:
    struct Edge
    {
        string destination;
        int weight;
    };
    struct vertice
    {
        string location;
        vector<Edge> directions;
    };
    Graph is a vector of vertice. Also when I am in the debugger, and I try to track the value of graph[i].location in the for loop, I get
    Code:
    graph[i].location    CXX0058: Error: overloaded operator not found
    Last edited by workisnotfun; 03-03-2013 at 09:35 PM.

  2. #2
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    Dam I think it might be something with my syntax because I changed
    Code:
    hp = (structnode*)(malloc(sizeof(structnode)));
    hp->location = graph[index].location;
    to
    Code:
    node head;    
    head.location = graph[index].location;
        hp=&head;
    and it gets through this part perfectly fine.

    Can someone explain what's going on? I actually came from a C point of view and trying to use C++, and it seems I don't even have to malloc, is that right? What are some of the things I am misunderstanding because it seems like the '->' use is different in C++ than in C? :/
    Last edited by workisnotfun; 03-03-2013 at 09:52 PM.

  3. #3
    and the hat of wrongness Salem's Avatar
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    A couple of comments.
    Code:
            struct node *hp;
            for(int i = 0; i<graph.size();i++)
    	{
    		cout << graph[i].location <<"=="<<graph[0].location<<"?\n";
    		if((graph[i].location).compare(start)==0)
    		{ 
    			index = i;
    			break;
    		}
    	}
        cout<<"Found "<< graph[index].location;
        //make starting nodes
        hp = (struct node*)(malloc(sizeof(struct node)));
        hp->location = graph[index].location;
    1. What guarantees that something will always be found. Or put another way, what garbage is in index if nothing is found.

    2. Don't use malloc in a C++ program, use new.
    hp = new node;

    It might be an idea to add a constructor as well, which will at least make sure you have some plausible data.
    Code:
    struct node{
        string location;
        int gval;
        int fval;
        node *next;
        node ( ) : next(0) {}
    };
    stahta01 likes this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  4. #4
    Registered User hk_mp5kpdw's Avatar
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    As mentioned by Salem you should be using new and not malloc here. The later does not construct the std::string data member location, it simply allocates sizeof(std::string) space for that member but does not actually build it. Using new will properly construct that piece of the struct. This is most certainly the reason for your crash.

    If your struct did not contain any objects needing to be constructed then using malloc would be allowable here but you'd still want to prefer new instead.
    Last edited by hk_mp5kpdw; 03-04-2013 at 07:59 AM.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Are you writing some sort of graph? In that case, you should probably look at smart pointers. Otherwise I have to question why you use new (or malloc) in the first place.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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