Problem using Templates in c++

[prakhs]

Hi,
Please help me debug this code. I made a Stack template with two parameters: type and size. I am using it two times: first in conversion from infix expression to postfix (here, <char,50> stack is used), then in evaluation of postfix expression (here, <long double, 50> stack is used).
There is an error while using 2nd stack. how to debug it.

#include <iostream>
#include <cctype>
#include <cmath>
using namespace std;

template <class T,int size>

class Stack {
private:
    int top;
    T *nodes;
public:
    Stack ();
    int empty (void);
    void push(T &);
    T pop(void);
    T pop(int &);

    ~Stack ();
};
template <class T, int size> Stack<T,size>::Stack ()
{
    top = -1;
    nodes = new T[size];
};
template <class T, int size> Stack<T,size>::~Stack ()
{
    delete nodes;
};
template <class T, int size> int Stack<T, size>::empty()
{
    if (top == -1)
        return 1;
    else
        return 0;
}
template <class T, int size> void Stack<T,size>::push(T & j)
{
    if (top == size - 1) {
        cout << "Stack overflow" << endl;
        return;
    }
    nodes[++top] = j;
};
template <class T, int size> T Stack<T,size>::pop(void)
{
    T p;
    if (empty ()) {
        cout << "Stack underflow" << endl;
        return p;
    }
    p = nodes[top--];
    return p;
};
//popandtest
template <class T, int size> T Stack<T,size>::pop(int & und)
{
    T p;
    if (empty ()) {
        und = 1;
        return p;
    }
    und = 0;
    p = nodes[top--];
    return p;
};


void postfix(char *infix, char *postr);
int isoperand(char op);
int isoperator(char op);
long double eval(char *postr);
long double oper(int sym, long double op1, long double op2);
int priority(char op);

int priority(char op) {
    if (op == '^' || op == '$')
        return 3;
    else if (op == '%' || op == '*' || op == '/')
        return 2;
    else if (op == '+' || op == '-')
        return 1;
    else
        return 0;
}


int isoperator(char op) {
    if (op == '^' || op == '$' || op == '%' || op == '/' || op == '*' || op == '+' || op == '-')
        return 1;
    else 
        return 0;
}

int isoperand(char op) {
    if (isalnum(op))
        return 1;
    else 
        return 0;
}

void postfix(char *infix, char *postr)
{
    int position, und;
    int outpos = 0;
    char topsymb;
    Stack<char,50> opstk;

    for (position = 0; infix[position] != '\0'; position++) {
        if (isoperand(infix[position]))
        {
            postr[outpos++] = infix[position];
        }
        else if (infix[position] == '(')
        {
            opstk.push(infix[position]);
        }
        else if (isoperator(infix[position]))
        {
            int prty = priority(infix[position]);
            topsymb = opstk.pop(und);
            while (!und && priority(topsymb) >= prty) {
                postr[outpos++] = topsymb;
                topsymb = opstk.pop(und);
            }
            if (!und)
                opstk.push(topsymb);
            opstk.push(infix[position]);
        }
        else if (infix[position] == ')')
        {
            while ((topsymb = opstk.pop()) != '(')
                postr[outpos++] = topsymb;
        }
    }

    while (!opstk.empty())
        postr[outpos++] = opstk.pop();

    postr[outpos] = '\0';
}

long double oper(int symb, long double op1, long double op2)
{
    switch (symb) {
        case '+':
            return op1+op2;
            break;
        case '-':
            return op1-op2;
            break;
        case '*':
            return op1*op2;
            break;
        case '/':
            return op1/op2;
            break;
        case '^': case '$':
            return pow(op1,op2);
        default: 
            cout << "Invalid operator\n";

    }
}

long double eval(char *postr)
{
    int c, position;
    long double opnd1, opnd2, value;
    Stack<long double, 50> opndstk;
    for (position = 0; (c = postr[position]) != '\0'; position++)
        if (isoperand(c))
            opndstk.push((long double) (c-'0'));
        else {
            opnd2 = opndstk.pop();
            opnd1 = opndstk.pop();
            value = oper(c, opnd1, opnd2);
            opndstk.push(value);
        }
    return (opndstk.pop());
}

int main(void)
{
    char in[250], post[250];
    long double res;
    cin >> in;
    cout << in << endl;
    postfix (in, post);
    res = eval(post);
    cout << res << endl;

    return 0;
}

[deceptikon]

There is an error while using 2nd stack. how to debug it.

Start by telling us the error and line. Here, I'll help. This error occurs on line 173:

'void Stack<long double,50>::push(T &)' : cannot convert parameter 2 from 'long double' to 'long double &'

And it has nothing to do with templates, you're passing an incompatible type. push() expects a reference to double, not a double value. So you'll need to either overload push() to take a value, change the reference to a const reference (recommended) because you don't modify it, or create a temporary variable to pass.

The recommended fix looks like this:

template <class T, int size> void Stack<T,size>::push(const T & j)
{
    if (top == size - 1) {
        cout << "Stack overflow" << endl;
        return;
    }
    nodes[++top] = j;
};

[prakhs]

Can you explain why const keyword works? The push function still needs a reference to double value. Only thing I know about const is that we cannot change the value of const variable.

[deceptikon]

Can you explain why const keyword works?

It's just a quirk of C++. Put relatively simply, a non-const reference cannot bind to a value because a value cannot be modified unless it resides in a variable. A const reference can bind to a value because it cannot be modified through the reference due to being const. The const reference doesn't need to assume that it's bound to a variable.

[prakhs]

Thank you very much :)