摘自Programming -- Principles and Practice Using C++ by Bjarne Stroustrup

calc.cc

/*
*Caculator Parser
*
*	expression
*		term
*		expression + term
*		expression - term
*
*	term
*		primary
*		term * primary
*		term / primary
*
*	primary
*		number
*		"("expression")"
*
*	number
*		float-pointing-literal
*/
#include "../std_lib_facilities.h"
class Token
{
  public:
    Token(char ch)
        : kind(ch), value(0){};
    Token(char ch, double val)
        : kind(ch), value(val){};
    char kind;
    double value;
};

class Token_stream
{
  public:
    Token_stream() : full(false), buffer(0){};
    Token get();
    void putback(Token c);

  private:
    bool full;
    Token buffer;
};

void Token_stream::putback(Token t)
{
    buffer = t;
    full = true;
}

Token Token_stream::get()
{
    if (full)
    {
        full = false;
        return buffer;
    }
    char ch;
    cin >> ch;
    switch (ch)
    {
    case '(':
    case ')':
    case '+':
    case '-':
    case '*':
    case '/':
    case 'q':
    case ';':
        return Token(ch);
    case '.':
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
        cin.putback(ch);
        double val;
        cin >> val;
        return Token('8', val);
    default:
        error("Bad Token");
    }
}

Token_stream ts;

double expression();

double primary()
{
    Token t = ts.get();
    switch (t.kind)
    {
    case '(':
    {
        double d = expression();
        t = ts.get();
        if (t.kind != ')')
            error("')' expected");
        return d;
    }

    case '8':
        return t.value;
    default:
        error("primary expected");
    }
}

double term()
{
    double left = primary();
    Token t = ts.get();
    while (true)
    {
        switch (t.kind)
        {
        case '*':
            left *= primary();
            t = ts.get();
            break;
        case '/':
        {
            double val = primary();
            if (val == 0)
                error("divide by zero");
            left /= val;
            t = ts.get();
            break;
        }
        default:
            ts.putback(t);
            return left;
        }
    }
}

double expression()
{
    double left = term();
    Token t = ts.get();
    while (true)
    {
        switch (t.kind)
        {
        case '+':
            left += term();
            t = ts.get();
            break;
        case '-':
            left -= term();
            t = ts.get();
            break;
        default:
            ts.putback(t);
            return left;
        }
    }
}

int main()
{
    cout << "welcome to our simple caculator." << endl;
    cout << "please enter expressions using float numbers." << endl;
    try
    {
        double val;
        while (cin)
        {
            Token t = ts.get();
            if (t.kind == 'q')
                break;
            if (t.kind == ';')
                cout << "=" << val << endl;
            else
                ts.putback(t);
            val = expression();
        }
        keep_window_open();
    }
    catch (exception &e)
    {
        cerr << e.what() << endl;
        keep_window_open();
        return 1;
    }
    catch (...)
    {
        cerr << "exception\n";
        keep_window_open();
        return 2;
    }
    return 0;
}