@Yash001 said in why definition is talking from .cpp file?:

why it is working if I will define the definition inside the class

Because then the compiler sees the definition when you compile your executable.

just now I try Q_DECL_EXPORT virtual QString GetExpParamTxt(QWidget*)const;. I got same error.

You did not read the link carefully... you have to export it when compiling the lib, import it when linking against as explained.

@raven-worx Thank you. I will use UTF-8.

@JonB Thank you for correct direction. I will modify my code and use it finished Signal for updating dialog box.