C++ 货币中的精度处理

应用场景

1、在处理港交所中的Decimal类型时，需要将Decimal类型的字段除以pow(10, 8),还原为Double类型。

2、当传输给OCG时还要将Double类型还原为Decimal类型。

代码实现

我们把Double转换为String，经过处理，然后在转换为Decimal。这样可以避免计算中的精度丢失。

参考比特币中的处理

static const int64 HKCOIN = 100000000;

std::string FormatMoney(const int64& n)
{
    // Note: not using straight sprintf here because we do NOT want
    // localized number formatting.
    int64_t n_abs = (n > 0 ? n : -n);
    int64_t quotient = n_abs/HKCOIN;
    int64_t remainder = n_abs%HKCOIN;
    std::string str = strprintf("%d.%08d", quotient, remainder);

    // Right-trim excess zeros before the decimal point:
    int nTrim = 0;
    for (int i = (int)str.size()-1; (str[i] == '0' && isdigit(str[i-2])); --i)
        ++nTrim;
    if (nTrim)
        str.erase(str.size()-nTrim, nTrim);

    if (n < 0)
        str.insert((unsigned int)0, 1, '-');
    return str;
}

bool ParseMoney(const std::string& str, int64& nRet)
{
    return ParseMoney(str.c_str(), nRet);
}


/**
 * Tests if the given character is a decimal digit.
 * @param[in] c     character to test
 * @return          true if the argument is a decimal digit; otherwise false.
 */
constexpr bool IsDigit(char c)
{
    return c >= '0' && c <= '9';
}

/**
 * Tests if the given character is a whitespace character. The whitespace characters
 * are: space, form-feed ('\f'), newline ('\n'), carriage return ('\r'), horizontal
 * tab ('\t'), and vertical tab ('\v').
 *
 * This function is locale independent. Under the C locale this function gives the
 * same result as std::isspace.
 *
 * @param[in] c     character to test
 * @return          true if the argument is a whitespace character; otherwise false
 */
constexpr inline bool IsSpace(char c) noexcept {
    return c == ' ' || c == '\f' || c == '\n' || c == '\r' || c == '\t' || c == '\v';
}


bool ParseMoney(const char* pszIn, int64& nRet)
{
    std::string strWhole;
    int64_t nUnits = 0;
    const char* p = pszIn;
    while (IsSpace(*p))
        p++;
    for (; *p; p++)
    {
        if (*p == '.')
        {
            p++;
            int64_t nMult = HKCOIN / 10;
            while (isdigit(*p) && (nMult > 0))
            {
                nUnits += nMult * (*p++ - '0');
                nMult /= 10;
            }
            break;
        }
        if (IsSpace(*p))
            break;
        if (!isdigit(*p))
            return false;
        strWhole.insert(strWhole.end(), *p);
    }
    for (; *p; p++)
        if (!IsSpace(*p))
            return false;
    if (strWhole.size() > 10) // guard against 63 bit overflow
        return false;
    if (nUnits < 0 || nUnits > HKCOIN)
        return false;
    int64_t nWhole = strtoll(strWhole.c_str(), nullptr, 10);
    int64 nValue = nWhole*COIN + nUnits;

    nRet = nValue;
    return true;
}


int main(int argc,char* argv[]) {
    string s = FormatMoney(1000000006);
    std::cout << s << endl;
    
    int64 n = 0;
    bool b = ParseMoney("10000.03100012", n);
    std::cout << n << endl;
    return 0;
}