CODEWITHSUNDEEP
c++pointerswhile-loopbase-classabstract-base-class
user3566909
user3566909

Reputation:

Pointer to base class lost in while loop, causing segfault. C++

This code uses a while loop to get user input and execute the appropriate command - I've reduced it to 2 commands for sake of brevity.
The Oblock object is created correctly (command "O"), as is the pointer to the base class. It appears that the calls to both objects work correctly as well. However, after returning to the while loop, the pointer to the object appears to be lost, and attempting to access its members (command "t") causes a segfault. I've included the example code below - my questions are afterwards.

#include<vector>
#include<iostream>
#include<string.h>

using namespace std;

class Tetramino {
    private:
        int squareSize;
        vector<string> myShape;
    public:
        void setValues(int size) {
            squareSize = size;
            myShape = vector<string> ((size*size), ".");
        }
        string getValues(int i) {
            return myShape[i];
        }
        int getSize() {
            return squareSize;
        }
};

class Oblock : public Tetramino {
    public:
    Oblock() {
        setValues(2);
    }
};

main () {
    string input;
    bool runProgram = true;
    Tetramino *pBlock;

    while (runProgram) {
        cin >> input;
        if (input == "O")
            {
                Oblock myBlock;
                cerr << "0thi: " << myBlock.getValues(0) << endl;
                Tetramino *pBlock = &myBlock;
                cerr << "0thi: " << pBlock->getValues(0) << endl;
            }
        if (input == "t")
            {
                cerr << "0thi: " << pBlock->getValues(0) << endl;
            }
        }
    return 0;
}

Thanks in advance for any advice! I searched for questions similar to this one and couldn't find one appropriate for my needs.

Upvotes: 1

Views: 609

Answers (4)

RamblingMad
RamblingMad

Reputation: 5518

Your pointer becomes garbage after myBlock goes out of scope (where the if statement closes) which will be causing the segmentation fault when you try to access it later

if(input == "O"){
    Oblock myBlock;
}
^^^^ right here myBlock becomes garbage

Also, if a user inputs "t" before "O" it will cause a segmentation fault because they will be trying to access an uninitialized pointer. You should probably get that looked at.

Upvotes: 0

PMF
PMF

Reputation: 17278

Answering your first question: Yes, the statement

Oblock yblock;

Creates an instance of Oblock on the stack. It is destroyed when the code leaves the respective block, so any pointers to it become invalid after that. To create an object that lives as long as you wish, use new to create the object on the heap.

Upvotes: 1

aschepler
aschepler

Reputation: 72431

 if (input == "O")
 {
     Oblock myBlock;
     cerr << "0thi: " << myBlock.getValues(0) << endl;
     Tetramino *pBlock = &myBlock;
     cerr << "0thi: " << pBlock->getValues(0) << endl;
 }

An object with automatic storage duration (commonly called a function-local variable) has a lifetime beginning after its declaration and ending at the end of the nearest enclosing block } token. So myBlock is destroyed at the end of this if statement, and can't be used again.

Also note that you have declared two different pointers named pBlock. Assigning the inner one does nothing to the earlier one, which is still uninitialized.

Upvotes: 1

littleadv
littleadv

Reputation: 20282

Tetramino *pBlock is local within its scope. You're shadowing over the one in main with the one within the if.

Also, myBlock is local and will be destructed - you'll have a dangling pointer. You should allocate with new (and delete...)

Instead of Tetramino *pBlock = &myBlock; do pBlock = new Oblock; when you handle the "O" input (and handle delete pBlock of the previous one).

Upvotes: 3

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