KB11558 - How to convert a string to a floating-point number in C++

SUMMARY

In an application developed with Microsoft C or C/C++, the sscanf() function is a good alternative to the atof() function to convert a string of digits into a floating-point number. If a string does not represent a valid number, atof() returns the value zero; sscanf() returns more useful error information. The application can use the error value from sscanf() with the matherr() function to perform error handling. The atof() function does not call matherr() unless an actual math exception occurs.

The text below presents two recommended methods to convert a string to a floating-point number.

Validate the string to convert prior to calling the atof() function. Make sure that the string does not contain any non- numeric characters and that the decimal point and sign characters are in the correct locations.
Use the sscanf() function. It is slower than the atof() function, but it provides better information when an error occurs.