Why Does `atof` Sometimes Return Zero When Converting `std::string` to `double`, and What Are the Alternatives?

Converting std::string to Double with atof

When attempting to convert a std::string to a double using the atof function, some programmers encounter an issue where it consistently returns zero. To address this, the following revised code is recommended:

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

The double colon (::) before atof specifies that we are calling the global function, which is necessary to resolve the issue.

Alternative Approaches

While atof is a valid option for string-to-double conversion, there are other approaches to consider:

• stringstream: This method is more versatile but has a slight performance penalty.
• boost::lexical_cast: This library function can also perform string-to-double conversion, but it incurs a similar performance hit.

Additional Considerations for Qt Projects

For Qt projects specifically, utilizing QString::toDouble() is recommended:

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

For optimal performance when dealing with const char*, QByteArray::toDouble() should be preferred.

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