This should do exactly what you want. You can test it live at: http://www.wowpanda.net/calc

It uses Reverse Polish Notation and supports:

• Operator precedence (5 + 5 * 5 = 30 not 50)
• Parens ((5 + 5) * 5 = 50)
• The following operators: +, -, *, /

EDIT: you ll probably want to remove the Abs() at the bottom; for my needs 0 - 5 should be 5 and not -5!

static bool Rpn(const string expression, vector<string> &output)
{
    output.clear();
    char *end;
    vector<string> operator_stack;
    bool expecting_operator = false;

    for (const char *ptr = expression.c_str(); *ptr; ++ptr) {
        if (IsSpace(*ptr))
            continue;

        /* Is it a number? */
        if (!expecting_operator) {
            double number = strtod(ptr, &end);
            if (end != ptr) {
                /* Okay, it s a number */
                output.push_back(boost::lexical_cast<string>(number));
                ptr = end - 1;
                expecting_operator = true;
                continue;
            }
        }

        if (*ptr ==  ( ) {
            operator_stack.push_back("(");
            expecting_operator = false;
            continue;
        }

        if (*ptr ==  ) ) {
            while (operator_stack.size() && operator_stack.back() != "(") {
                output.push_back(operator_stack.back());
                operator_stack.pop_back();
            }

            if (!operator_stack.size())
                return false; /* Mismatched parenthesis */

            expecting_operator = true;
            operator_stack.pop_back(); /* Pop  (  */
            continue;
        }

        if (*ptr ==  +  || *ptr ==  - ) {
            while (operator_stack.size() && IsMathOperator(operator_stack.back())) {
                output.push_back(operator_stack.back());
                operator_stack.pop_back();
            }

            operator_stack.push_back(boost::lexical_cast<string>(*ptr));
            expecting_operator = false;
            continue;
        }

        if (*ptr ==  *  || *ptr ==  / ) {
            while (operator_stack.size() && (operator_stack.back() == "*" || operator_stack.back() == "/")) {
                output.push_back(operator_stack.back());
                operator_stack.pop_back();
            }

            operator_stack.push_back(boost::lexical_cast<string>(*ptr));
            expecting_operator = false;
            continue;
        }

        /* Error */
        return false;
    }

    while (operator_stack.size()) {
        if (!IsMathOperator(operator_stack.back()))
            return false;

        output.push_back(operator_stack.back());
        operator_stack.pop_back();
    }

    return true;
} // Rpn

/***************************************************************************************/

bool Calc(const string expression, double &output)
{
    vector<string> rpn;

    if (!Rpn(expression, rpn))
        return false;

    vector<double> tmp;
    for (size_t i = 0; i < rpn.size(); ++i) {
        if (IsMathOperator(rpn[i])) {
            if (tmp.size() < 2)
                return false;
            double two = tmp.back();
            tmp.pop_back();
            double one = tmp.back();
            tmp.pop_back();
            double result;

            switch (rpn[i][0]) {
                case  * :
                    result = one * two;
                    break;

                case  / :
                    result = one / two;
                    break;

                case  + :
                    result = one + two;
                    break;

                case  - :
                    result = one - two;
                    break;

                default:
                    return false;
            }

            tmp.push_back(result);
            continue;
        }

        tmp.push_back(atof(rpn[i].c_str()));
        continue;
    }

    if (tmp.size() != 1)
        return false;

    output = Abs(tmp.back());
    return true;
} // Calc

/***************************************************************************************/

boost::spirit comes with a calculator example which would do what you need: http://www.boost.org/doc/libs/1_33_1/libs/spirit/example/fundamental/ast_calc.cpp

muParser is written in C++ and does just what you need.

C++ in Action, in addition to being a great book on C++, includes a fully working calculator, doing what you need (and actually much more). And the book is available for free online