how can I convert a C-String into long double using _atold() function in C++ or any other function?

Question

// See the code below and help me why i did not getting the right result. Or suggest any other C++ function to convert a C-string like "$567,789,675.89" into long double

  long double mstold( char s[] )
{
    int len = strlen(s);
    long double cash;
    int n=0;
    char amount[ 100 ];
for( int i=0; i<len; i++)                       // for copying the passed C-String into another C-string
  {
    amount[n] = s[i];
      n++;
    if( s[i] == '$' || s[i] == ',')     // Because the C-String has been passed in format: "$567,789,564.987"
        n--;
  }

   cash = _atold( amount );           // This does not gives the right result
   return cash;
 }

Answer 1

Use strtold() function, since _atold() is a non standard function. I am posting the code which works in compiler explorer. You were not terminating amount array with '\0'. Perhaps that's the reason _atold not worked.

#include <cstdlib>
#include <iostream>
#include <cstring>


using namespace std;

long double mstold(const char* s)
{
    int len = strlen(s);
    long double cash;
    int n = 0;
    char* amount = new char[len+1];
    for (int i = 0; i<len; i++)                       // for copying the passed C-String into another C-string
    {
        amount[n] = s[i];
        n++;
        if (s[i] == '$' || s[i] == ',')     // Because the C-String has been passed in format: "$567,789,564.987"
            n--;
    }
    amount[n] = '\0';
    cash = strtold(amount, NULL);           // This does not gives the right result
    delete[] amount;
    return cash;
}

int main()
{
    long double cash = mstold("$567,789,675.89");
    std::cout << cash << std::endl;
}

Answer 2

First note. Please do not use C-Style strings. In C++ we use std::string. Anyway, also C-style strings will do and can be converted automatically.

Then, for newbies it is the best to transform the input monetary-string to a number-string with just one decimal digit and then use function stold for conversion. You may read here about it.

But in the real C++ world, you would do 2 things:

• use dedicated C++ facilities
• use localization

Unfortunately this is a rather complex topic and you need a while to understand.

You need to read about the localization library. Here you will learn about 2 major concepts:

1. locales
2. facets

In general textual representation of dates, monetary values or number formats are governed by regional or cultural conventions. All this properties are contained in a std::locale object.

But the std::locale does not offer much functionality. The real localization facilities are offered in the form of facets. And a std::locale encapsulates several facets. And one of them is about the monetary formatting.

You really can do many things with that and in the end get fully customized behaviour. But, as said, not that easy to understand.

I will use the std::money_get class in my below example.

Please note. This will convert your number into units, without a fraction. In financial calculations we basically should not use fractions, because double or even long double cannot store all "reaal" values". Please read about this as well.

Anyway. I will show you an example how such a monetary value would be converted in C++. You maybe shocked by the complexity, but flexibility has its price . . .

Please see:

#include <iostream>
#include <iomanip>
#include <limits>
#include <string>
#include <locale>
#include <sstream>

int main() {
    // Input String
    char valueAsCString[] = "$567,789,675.89";

    // Put it in an istringstream for formatted extraction
    std::istringstream iss{ valueAsCString };

    // Assume US currency format (On Posix machines, please use "en-US") and set it for the stream
    std::locale myLocale("en_US");
    iss.imbue(myLocale);
    
    // Assume that everthing is good
    std::ios_base::iostate ioError{ std::ios_base::goodbit };

    // Here we will get the value in UNITS, so without a fraction!!!
    long double value{};

    // Parse the money string and get the result
    std::use_facet<std::money_get<char>>(myLocale).get(std::istreambuf_iterator<char>(iss), {}, false, iss, ioError, value);

    // Check Error state
    iss.setstate(ioError);
    if (iss)
        // Show result
        std::cout << std::setprecision(std::numeric_limits<long double>::digits10 + 1) << std::setw(25) << value / 100 << '\n';
    else
        std::cerr << "\nError during conversion\n";
}