Why Does `atof` Fail to Convert `std::string` to `double` in Qt, and What Are the Alternatives?

std::string to Double Conversion Issue: Using atof with Qt Projects

Converting a std::string to a double using the atof function can be problematic in Qt projects. Let's examine the code and explore alternative approaches:

The code provided:

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

attempts to convert a std::string to a double. However, it returns zero. This issue arises because QStrings, commonly used in Qt, are passed as const char*.

To resolve this, explicitly cast the QString to double:

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

Alternatively, QByteArray::toDouble can be used for faster conversions when working with const char*.

For non-Qt projects, the following syntax is valid:

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

While stringstream or boost::lexical_cast can also perform the conversion, they incur a performance penalty.

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