How to use private static variable in a class? error: collect2: ld returned 1 exit

[furskytl]

I am an stranger to Qt.I use a private static variable in a class,like this:
private:
static QString lastFilePath ;
However it doesn't work with the complie output "collect2: ld returned 1 exit status"!If I delete the keyword "static",it can run normally!What's wrong with my code?How to define a static member in a class in Qt?Thanks!