Question

QIODevice and the related classes use qint64 for positions and sizes which is a signed datatype. Is there a need to express negative values? Because otherwise the 8 bytes of such a type could be used to express greater sizes, couldn’t they?

Answer 1

An error value of -1 is returned for several functions in QIODevice. Qt often uses a C style of error handing using return values to avoid the need for a compiler or platform that supports the use of C++ exceptions. It is important to check for these error codes.

From the manual:

• QIODevice::write and QIODevice::writeData

  Returns the number of bytes that were actually written, or -1 if an error occurred.

• QIODevice::read(char*,qint64)

  If an error occurs, … this function returns -1.

• QIODevice::readData(char*,qint64)

  … and returns the number of bytes read or -1 if an error occurred.

• QIODevice::peek(char*,qint64)

  If an error occurs, … this function returns -1.