Cannot write a function pointer into a class implementation

[tonnot]

I have a MainWindow class with ther .h and cpp files.
Compiler tells me :

argument of type 'void (MainWindow:(long int)' does not match 'void (*)(long int)'

I have tried all combinations, with class prefix, without it,.... without success.

void MainWindow::update1(long value){ }
 
void MainWindow::on_pushButton_6_clicked()
{
    void (*pFunc)(long);
    pFunc = update1;        // here is where I have the error.
 
}

void MainWindow::update1(long value){ }

void MainWindow::on_pushButton_6_clicked()
{
    void (*pFunc)(long);
    pFunc = update1;        // here is where I have the error.

}

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Any help ? Thanks