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

Why std::string to Double Conversion with atof Returns Zero

When attempting to convert a std::string to double using atof, users may encounter a situation where the returned value is consistently zero. To understand the cause, let's delve into the related code and explore alternative approaches to resolve this issue.

atof Usage in std::string to Double Conversion

The atof function is commonly used to convert a null-terminated character array to a double-precision floating-point value. However, when using std::string, the issue arises because std::string is not a null-terminated character array. This results in atof being unable to properly parse the string and returning zero.

Proper atof Usage with std::string

To resolve this issue, one must convert the std::string to a null-terminated character array before using atof. This can be achieved by using the c_str() member function of std::string. The correct code should resemble:

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

In this scenario, ::atof() ensures that the global scope of the function is used instead of the local scope, where atof is undefined when using Qt.

Alternative Conversion Methods

Aside from using atof, other methods exist for converting a std::string to double, such as:

• std::stringstream:

  std::stringstream ss(num);
  double temp;
  ss >> temp;

• boost::lexical_cast:

  #include <boost/lexical_cast.hpp>
  double temp = boost::lexical_cast<double>(num);

However, these methods may incur performance penalties compared to using atof with the appropriate string conversion.

Specific Considerations for Qt Projects

If working with a Qt project, QString provides a convenient toDouble() method that facilitates string to double conversion more efficiently than using std::string. For example:

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

Additionally, for input data in the form of a const char*, QByteArray::toDouble offers even better performance.

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