How to check if a string can be converted to double in C++?

Question

I have a string which can be a number (even a float or double type, not only integer), and it can be also a word which is non-numeric.

I would like to check if this string can be converted into double, and if so, then I would like to do the conversion. In case of a non-numeric string, I want different behaviour.

I have tried this:

double tmp;
string str;
stringstream ss;

ss << str;
ss >> tmp;

if (ss.fail())
{
    // non-numeric string
}
else
{
    // string to double conversion is successful
}

The problem with this code is that ss.fail() is always true, even if tmp contains the right value.

There is a function called atof() which converts string to double, but this is not suitable for me, becouse it returns 0.0 value if the input string is non-numeric. This way I cannot make difference between a non-numeric and a zero input value.

Answer 1

What about std::stod? It will throw std::out_of_range when it can't perform the conversion.

try
{
    double value = std::stod(input_string);
    std::cout << "Converted string to a value of " << value << std::endl;
}
catch(std::exception& e)
{
    std::cout << "Could not convert string to double" << std::endl;
}

I haven't tried to compile it, but you should get the idea.

Answer 2

You can use this function if your input string have type from std::string (it works on windows and unix systems):

#include <stdlib.h>
#include <string>
/**
* @brief checkIsDouble - check inputString is double and if true return double result
* @param inputString - string for checking
* @param result - return double value
* @return true if string is double, false if not
*/
bool checkIsDouble(string inputString, double &result) {
    char* end;
    result = strtod(inputString.c_str(), &end);
    if (end == inputString.c_str() || *end != '\0') return false;
    return true;
}