C++ but it fails for example test case 1 but passed for second one

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class TransactionalStack
{
private:
    stack<TransactionalStack*> transactions;
    stack<int> content;

public:
    TransactionalStack() : TransactionalStack(false)
    {}

    TransactionalStack(bool internal)
    {

        if(!internal)
        {
            transactions.push(this);
//            TransactionalStack* top = transactions.top();
//            top->content.push(5);
        }
    }

    void push(int value)
    {
        TransactionalStack* top = transactions.top();
        top->content.push(value);
    }

    int top()
    {
        stack<int> cn ;
        cn = transactions.top()->content;
        return cn.empty() ? 0 : cn.top();
    }

    int pop()
    {
        stack<int> cn;
        cn = transactions.top()->content;
        if(cn.empty())
            return 0;
        int t = cn.top();
        cn.pop();
        return t;
    }

    /*void begin()
    {
        TransactionalStack* newTransaction = new TransactionalStack();
        newTransaction->content = content; // assignment operator automatically copies
        transactions.push(newTransaction);
    }*/

    bool rollback()
    {
        TransactionalStack* lastTransaction = transactions.top();
        if(lastTransaction != this)
        {
//            lastTransaction = transactions.top();
            transactions.pop();
//            transactions.top()->content = lastTransaction->content;
            return true;
        }
        return false;
    }

    bool commit()
    {
        TransactionalStack* lastTransaction = transactions.top();
        if(lastTransaction != this)
        {
            lastTransaction = transactions.top();
            transactions.pop();
            transactions.top()->content = lastTransaction->content;
            return true;
        }
        return false;
    }
};

int main()
{
    TransactionalStack* ts = new TransactionalStack();
    /*ts->push(4);
    ts->begin();
    ts->push(7);
    ts->begin();
    ts->push(2);
    assert(ts->rollback() == true);
    assert(ts->top() == 7);
    ts->begin();
    ts->push(10);
    assert(ts->commit() == true);
    assert(ts->top() == 10);
    assert(ts->rollback() == true);
    assert(ts->top() ==4);
    assert(ts->commit() == false);
    return 0;*/


    ts->push(5);
    ts->push(2); // stack: [5,2]
    assert(ts->top() == 2);
    ts->pop(); // stack: [5]
    assert(ts->top() == 5);

    TransactionalStack* ts1 = new TransactionalStack();
    assert(ts1->top() == 0); // top of an empty stack is 0
    ts1->pop();
}

What I have tried:

Debugging fails for test case

ts->push(5);
    ts->push(2); // stack: [5,2]
    assert(ts->top() == 2);
    ts->pop(); // stack: [5]
    assert(ts->top() == 5);

    TransactionalStack* ts1 = new TransactionalStack();
    assert(ts1->top() == 0); // top of an empty stack is 0
    ts1->pop();

Posted 3-Mar-21 12:46pm

Abhinav Dutt

Updated 3-Mar-21 16:42pm

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Rick York · Answer 1 · 2021-03-03T16:42:00

I think I see your problem. It has to do with your methods :

C++

int top()
{
    stack<int> cn ;          // <----------------------
    cn = transactions.top()->content;
    return cn.empty() ? 0 : cn.top();
}

int pop()
{
    stack<int> cn;           // <----------------------
    cn = transactions.top()->content;
    if(cn.empty())
        return 0;
    int t = cn.top();
    cn.pop();
    return t;
}

The problem has to do with what the comments point at. You are creating a copy of the content stack in both of those. That is not a good idea at all. Plus, in the case of pop, you are popping from the copy, not your actual stack. It would be better to make that a reference so the operations are performed on the actual members of the object and not copies. Here is how that could look using references.

C++

using stackint = std::stack< int >;

int top()
{
    stackint & cn = transactions.top()->content;
    return cn.empty() ? 0 : cn.top();
}

int pop()
{
    stackint & cn = transactions.top()->content;
    if(cn.empty())
        return 0;
    int t = cn.top();
    cn.pop();
    return t;
}

C++ but it fails for example test case 1 but passed for second one

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C++ but it fails for example test case 1 but passed for second one

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