Why Does `atof` Return Zero When Converting a `std::string` to a `double`?

Troubleshooting atof Conversion Issues

In an attempt to convert a std::string to double using atof, a developer encounters an unexpected issue where the conversion consistently returns zero. This error can be frustrating, especially for users who have followed the standard syntax.

The problematic code in question:

std::string num = "0.6";
double temp = (double)atof(num.c_str());

To resolve this issue, it's crucial to remember that atof requires a const char instead of a std::string as its input. This means that we need to convert the std::string to a const char using the c_str() method. However, in the provided code snippet, the c_str() method is incorrectly enclosed in parentheses, which is not necessary. The correct syntax should be:

std::string num = "0.6";
double temp = ::atof(num.c_str());

This modification allows the atof function to correctly parse the string and convert it to a double.

For users working with Qt projects, an alternative approach is to use the QString::toDouble() method, which is designed specifically for converting QStrings to doubles. This method is typically faster than atof when dealing with QString inputs.

QString winOpacity("0.6");
double temp = winOpacity.toDouble();

It's worth noting that QByteArray::toDouble may offer better performance when working with const char* inputs.

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