Perchè non mi risolve l'espressione ???

Salve, ho fatto un programmino che dovrebbe risolvere una banale espressione usando lo stack ma una volta inserita l'espressione mi da risultato sempre 6 , perchè???

https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-prn2/v/t35.0-12/10287603_4180198519919_838562371_o.jpg?oh=1ccb0f6985211372f08825c7e325dead&oe=535FE882&__gda__=1398778984_61afbbea6a962bca4196f0c0ceba6b73

il codice è:
expression.h

#include <iostream>
#include <stack>
using namespace std;

double insertExpr(istream& ins);
void calculateExpr(stack<double> &numbers, stack<char> &operations);

arithexpression.cpp

#include "expression.h"
#include <iostream>
#include <cstdlib>
#include <stack>
#include <cctype>
#include <cstring>

using namespace std;

double insertExpr(istream& ins)
{
	stack<char> operations;
	stack<double> numbers;
	double number;
	char operation;
	while(ins && ins.peek()!='\n')
	{
		if(isdigit(ins.peek()) || ins.peek()=='.')
				{
					ins>>number;
					numbers.push(number);
				}
		else if(strchr("+-*/",ins.peek())!=NULL)
		{
			ins>>operation;
			operations.push(operation);
		}
		else if(ins.peek()==')')
		{
			ins.ignore();
			calculateExpr(numbers,operations);
		}
		else if(ins.peek()=='(' || ins.peek()==' ')
			ins.ignore();
	}
	return numbers.top();
}

void calculateExpr(stack<double> &numbers, stack<char> &operations)
{
	double operand1, operand2;
	operand2=numbers.top(); //si trova prima il secondo operando
	numbers.pop();
	operand1=numbers.top(); //poi il primo
	numbers.pop();
	switch(operations.top())
	{
		case '+': numbers.push(operand1+operand2);
				break;
		case '-': numbers.push(operand1-operand2);
				break;
		case '*': numbers.push(operand1*operand2);
				break;
		case '/': numbers.push(operand1/operand2);
				break;
	}
	operations.pop();
}

expression.cpp

#include "expression.h"
#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
	double risultato;
	cout<<"Espressione:\n";
	risultato=insertExpr(cin);
	cout<<"Risultato: "<<risultato<<"\n";

	return 0;
}