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

Trouble Converting from std::string to double Using atof?

When attempting to convert a std::string to a floating-point type such as float or double using the atof function, you may encounter unexpected results. Here's a common issue and its solution:

The Issue: Zero Conversion

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

In the above code, the atof function returns zero instead of the expected value of 0.6. This occurs because atof is a C function that expects a C-style string (char *) as its argument. However, num.c_str() returns a pointer to the first character in the std::string object, which is an object type.

Solution

To resolve this issue, you should pass a C-style string directly to atof. Here's the corrected code:

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

The double colon (::) before atof indicates that the atof function is a global function declared in the std namespace.

Alternatives

While the above solution using atof is valid, it's worth noting that there are alternative methods that may provide additional functionality or performance benefits. For example:

• stringstream: This provides a stream-based interface for converting between string and numeric types.
• boost::lexical_cast: This library provides a more type-safe way to perform string conversions.

However, it's important to evaluate the performance implications of these alternative methods, as they may come with additional overhead compared to using atof.

Note for Qt Users

If your project uses the Qt framework, you can take advantage of the built-in QByteArray::toDouble method. It's typically faster for converting from const char* data than using std::stringstream.

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

The above is the detailed content of Why Does `atof` Return Zero When Converting a `std::string` to `double`?. For more information, please follow other related articles on the PHP Chinese website!