样例三：运算器（Calculator）

#include <stack>
#include <tpl/RegExp.h>
#include <tpl/ext/Calculator.h>
#include <cmath>
 
using namespace tpl;
 
void calculate2()
{
    typedef SimpleImplementation impl;
 
    // ---- define rules ---- 
    impl::Allocator alloc; 
    std::stack<double> stk; 
    impl::Grammar::Var rFactor; 
    impl::Grammar rMul( alloc, '*' + rFactor/calc<std::multiplies>(stk) );
    impl::Grammar rDiv( alloc, '/' + rFactor/calc<std::divides>(stk) );
    impl::Grammar rTerm( alloc, rFactor + *(rMul | rDiv) ); 
    impl::Grammar rAdd( alloc, '+' + rTerm/calc<std::plus>(stk) );
    impl::Grammar rSub( alloc, '-' + rTerm/calc<std::minus>(stk) );
    impl::Grammar rExpr( alloc, rTerm + *(rAdd | rSub) );
 
    impl::Rule rFun( alloc, 
        "sin"/calc(stk, sin) | "cos"/calc(stk, cos) | "pow"/calc(stk, pow) );
 
    rFactor.assign( alloc, 
        real()/assign(stk) |
        '-' + rFactor/calc<std::negate>(stk) |
        '(' + rExpr + ')' |
        (gr(c_symbol()) + '(' + rExpr % ',' + ')')/(gr(rFun) + '(') |
        '+' + rFactor );
 
    // ---- do match ----
 
    for (;;)
    {
        std::string strExp;
        std::cout << "input an expression (q to quit): ";
        if (!std::getline(std::cin, strExp) || strExp == "q") {
            std::cout << '\n';
            break;
        }
 
        try {
            while ( !stk.empty() )
                stk.pop();
            if ( !impl::match(strExp.c_str(), rExpr + eos(), skipws()) )
                std::cout << ">>> ERROR: invalid expression!\n";
            else
                std::cout << stk.top() << "\n";
        }
        catch (const std::logic_error& e) {
            std::cout << ">>> ERROR: " << e.what() << "\n";
        }
    }
}
 
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