Evaluate Math Expression from Strings in C++

#include <bits/stdc++.h>
using namespace std;

// from this site: https://www.geeksforgeeks.org/expression-evaluation/

int precedence(char op) {
    if (op == '+' || op == '-') {
        return 1;
    }
    if (op == '*' || op == '/') {
        return 2;
    }
    return 0;
}

double apply_op(double a, double b, char op) {
    switch (op) {
        case '+': {
            return a + b;
            break;
        }
        case '-': {
            return a - b;
            break;
        }
        case '*': {
            return a * b;
            break;
        }
        case '/': {
            return a / b;
            break;
        }
    }
}

int evaluate(string tokens) {
    stack<double> values;
    stack<char> ops;
    for (int i = 0; i < (int) tokens.size(); i++) {
        if (tokens[i] == ' ') {
            continue;
        } else if (tokens[i] == '(') {
            ops.push(tokens[i]);
        } else if (isdigit(tokens[i])) {
            double val = 0;
            while (i < (int) tokens.size() && isdigit(tokens[i])) {
                val = (val * 10) + (tokens[i] - '0');
                i++;
            }
            values.push(val);
            i--;
        } else if (tokens[i] == ')') {
            while (!ops.empty() && ops.top() != '(') {
                double val2 = values.top();
                values.pop();
                double val1 = values.top();
                values.pop();
                char op = ops.top();
                ops.pop();
                values.push(apply_op(val1, val2, op));
            }
            if (!ops.empty()) {
                ops.pop();
            }
        } else {
            while (!ops.empty() && precedence(ops.top()) >= precedence(tokens[i])) {
                double val2 = values.top();
                values.pop();
                double val1 = values.top();
                values.pop();
                char op = ops.top();
                ops.pop();
                values.push(apply_op(val1, val2, op));
            }
            ops.push(tokens[i]);
        }
    }
    while (!ops.empty()) {
        double val2 = values.top();
        values.pop();
        double val1 = values.top();
        values.pop();
        char op = ops.top();
        ops.pop();
        values.push(apply_op(val1, val2, op));
    }
    return values.top();
}

int main() {
    cout << evaluate("((9+(5-3)*4)/2)*8") << "\n";
}