写一个c++程序代码，以实现用户输入两个数字（二十以内的整数）的英文单词，计算出二者乘积，并输出结果数字及结果的英文单词。要求：使用string类完成，可完成多次计算。

以下是一个实现上述功能的C++程序代码：

#include <iostream>
#include <string>
using namespace std;

string numToWord(int num) {
    string ones[] = {"", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
    string tens[] = {"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
    string teens[] = {"ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};

    if (num < 10) {
        return ones[num];
    } else if (num >= 10 && num < 20) {
        return teens[num - 10];
    } else if (num >= 20 && num < 100) {
        return tens[num / 10] + (num % 10 != 0 ? " " + ones[num % 10] : "");
    } else {
        return "out of range";
    }
}

int wordToNum(string word) {
    string ones[] = {"", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
    string tens[] = {"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
    string teens[] = {"ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};

    if (word == "zero") {
        return 0;
    } else if (word == "one") {
        return 1;
    } else if (word == "two") {
        return 2;
    } else if (word == "three") {
        return 3;
    } else if (word == "four") {
        return 4;
    } else if (word == "five") {
        return 5;
    } else if (word == "six") {
        return 6;
    } else if (word == "seven") {
        return 7;
    } else if (word == "eight") {
        return 8;
    } else if (word == "nine") {
        return 9;
    } else {
        for (int i = 0; i < 10; i++) {
            if (word == teens[i]) {
                return 10 + i;
            }
        }

        for (int i = 2; i < 10; i++) {
            if (word == tens[i]) {
                return i * 10;
            }
        }

        return -1; // not a valid word
    }
}

int main() {
    string word1, word2;
    int num1, num2;

    while (true) {
        cout << "Enter two numbers (up to 20): ";
        cin >> word1 >> word2;

        num1 = wordToNum(word1);
        num2 = wordToNum(word2);

        if (num1 == -1 || num2 == -1) {
            cout << "Invalid input. Please enter valid numbers." << endl;
        } else {
            int product = num1 * num2;
            string productWord = numToWord(product);

            cout << "Product: " << product << " (" << productWord << ")" << endl;
        }
    }

    return 0;
}

程序中使用了两个函数：`numToWord` 和 `wordToNum`，分别用于将数字转换成英文单词和将英文单词转换成数字。在 `main` 函数中，使用循环不断接收用户输入，并调用上述两个函数计算乘积并输出结果。如果用户输入的单词不是有效的数字单词，则程序会提示用户重新输入。