运用栈实现表达式求值（+，-，*，/运算）

#include <iostream>
#include <string>
#include <cassert>
#include <iomanip>
using namespace std;
template<typename Type> class Stack {
private:
    Type *elements;
    int max_size, top_index;
public:
    Stack(int length_input) {
        elements = new Type[length_input];
        max_size = length_input;
        top_index = -1;
    }
    ~Stack() {
        delete[] elements;
    }
    bool push(const Type &element) {
        if (top_index >= max_size - 1) {
            return false;
        }
        top_index++;
        elements[top_index] = element;
        return true;
    }
    bool pop() {
        if (top_index < 0) {
            return false;
        }
        top_index--;
        return true;
    }
    Type top() {
        assert(top_index >= 0);
        return elements[top_index];
    }
    bool empty() {
        return top_index < 0;
    }
};

static char pre[128] = { 0 };

void setpre()
{
    pre['+'] = 1;
    pre['-'] = 1;
    pre['*'] = 2;
    pre['/'] = 2;
}

bool precede(char op1, char op2) {
    return pre[op1] > pre[op2] ? true : false;
}
double operate(char theta, double a, double b) {
    switch (theta) {
        case '-': return b - a; break;
        case '+': return a + b; break;
        case '*': return a * b; break;
        case '/': return (b * 1.0) / a; break;
    }
}
void calc(Stack<double> &numbers, Stack<char> &operators) {
    double a = numbers.top();
    numbers.pop();
    double b = numbers.top();
    numbers.pop();
    numbers.push(operate(operators.top(), a, b));
    operators.pop();
}
int main() {
    int n;
    setpre();
    cin >> n;
    Stack<double> numbers(n);
    Stack<char> operators(n);
    string buffer;
    cin >> buffer;
    int i = 0;
    while (i < n) {
        if (isdigit(buffer[i])) {
            numbers.push(buffer[i] - '0');
            i++;
        } else {
            if (operators.empty() || precede(buffer[i], operators.top())) {
                operators.push(buffer[i]);
                i++;
            } else {
                calc(numbers, operators);
            }
        }
    }
    while (!operators.empty()) {
        calc(numbers, operators);
    }
    cout << fixed << setprecision(3) << numbers.top() << endl;
    return 0;
}