Is this the correct approach to do input validation with floating point values?

After spending a good amount of time researching input validation, I combined a few ideas and came up with this:

Function to check a string for a valid double...

bool isDouble(double& destination, string& source)
{ // 64 bit
  bool goodValue = false;

  if (!source.empty()) {
    errno = 0;
    char *garbage = nullptr;
    destination = strtod(source.c_str(), &garbage);

    if (*garbage == '\0' && errno != ERANGE)
      goodValue = true;
  }

  return goodValue;
}

Function to check a string for a valid 32 bit integer...

bool isLong(long& destination, string& source)
{ // 32 bit (base 10)
  const short BASE = 10;
  bool goodValue = false;

  if (!source.empty()) {
    errno = 0;
    char* garbage = nullptr;
    destination = strtol(source.c_str(), &garbage, BASE);

    if (*garbage == '\0' && errno != ERANGE)
      goodValue = true;
  }

  return goodValue;
}

Sample Implementation

using namespace std;

int main() {
  string buffer;
  double value;

  cout << "Enter a value: ";
  getline(cin, buffer, '\n');

  if (isDouble(value, buffer))
    cout << "Value: " << value << endl;
  else
    cout << "E开发者_如何学CRROR: Invalid input\n";

  return 0;
}

Can anyone comment on if I am overlooking anything with this approach?

I'm not sure about "the" correct way, but it's certainly not how I'd do it. First and probably most obvious, this chunk of code:

    for (i = 0, d = 0; i < BUFFSIZE && buffer[i] != 0 && buffer[i] >= '0' &&
             buffer[i] <= '9' || (buffer[i] == '.' && d == 0); i++)
            if (buffer[i] == '.') ++d;

is duplicated in a couple of places (at least I think the other instance is identical, and probably should be anyway).

Second, you don't appear to allow numbers like 1e23 or -1.2, which are usually accepted as floating point.

Offhand, I think I'd use strtod to attempt to convert the input. You can use its second parameter to detect whether a conversion reached the end of the input string (if not, you'll know at least part of the input wasn't accepted). You'll then (apparently) want to check that the returned value was in the desired range.

Perhaps the strtod() function can be of help here, as it tells you how much has been converted:

const char * buf = get_raw_data(); // somehow

char * endptr;
const double value = strtod(buf, &endptr);

if (endptr != buf + std::strlen(buf))  { /* ... error ... */ }

As the source for buf you could tokenize your input with std::string token; std::cin >> token; or something like that and use token.c_str().

If it's an exercise you want, that's understandable. But otherwise, you can use istringstream to avoid reinventing the wheel:

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

int main () {

  int n,val;
  string stringvalues;

  stringvalues = "3.14159254 f01fhasfljk";
  istringstream iss(stringvalues);

  float x,y;
  iss >> x;

  cout << x * 2 << endl;

  iss >> y;
  if ( ! iss.fail() )
    {
      cout << y * 2 << endl;
    }
  else
    {
      cout << "second value not a number..." << endl;
    }

  return 0;
}