Testing if a string contains a numeric value?

#include <string>
#include <cctype>
string s;
...
...isdigit(s[i])... 
// i-th character will be promoted to int...

#include <string>
#include <sstream>

int main()
{
  std::string test("1234.56");
  std::istringstream inpStream(test);
  double inpValue = 0.0;
  if (inpStream >> inpValue)
  {
    // ... Success!!  test is a number.
  }
  else
  {
    // ... Failure!!  test is not a number.
  }
}

#include <iostream>
// iostream for test only...
#include <string>
#include <cstring>

using namespace std;

namespace {
	const char* spaces = " \t";
	const char* digits = "0123456789";
}

int IntLen(const char* cstr)
{
  int	k, n = 0;
  if (cstr)
  {
	n = strspn(cstr,spaces);
	cstr += n;
	if (*cstr == '-' || *cstr == '+')
		++cstr, ++n;
	k = strspn(cstr,digits);
	n = k?n+k:0;
  }
  return n;
}
/// <int>::[spaces][+|-]<digits>[garbage]
inline bool isInt(const char* cstr)
{
  return IntLen(cstr) > 0;
}

inline bool isInt(const string& s)
{
  return isInt(s.c_str());
}

/// <number>::[spaces]<int>[e|E[+|-]<uint>][garbage]
bool isNumber(const char* cstr)
{
  int	n = IntLen(cstr);
  if (n)
  {
	cstr += n;
	if (*cstr == 'e' || *cstr == 'E')
	   n = strspn(++cstr,digits);
  }
  return n > 0;
}

inline bool isNumber(const string& s)
{
  return isNumber(s.c_str());
}

inline void Test(const string& s)
{
  cout << isNumber(s) << endl;
}

int main()
{
  char line[80];
  while (cout << "Number:",
         cin.getline(line,sizeof line,'\n')
  ) Test(line);
  return 0;
}