error: ISO C++ forbids declaration of ‘mainScreenDLG’ with no type

[GreenScape]

read carefully:

declaration of ‘mainScreenDLG’ with no type

only constructors and destructors has no return type, you just need to add void (or other return type) before mainScreenDLG:

void mainScreenDLG(QDialog *parent = 0);

void mainScreenDLG(QDialog *parent = 0);

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Fine, but still, delete the "::", they do not belong to that place.

they do! they tell to compiler namespace where QDialog declared. in this case the do nothing beacuse QDialog in global scope.