how to set up Fl_Slider by value from Fl_Input

Question

I try to read value from Fl_Input and set up Fl_Slider by using code below but something is not working! I'm not sure about the operation

Thanks for help!

FL_Value_Input* vinput;

void set_slider(FL_Widget* w, void* v)
{
Fl_Slider* slider = (Fl_Slider*)w;
slider->value(vinput->value());
}


int main(){


    Fl_Window *window = new Fl_Window(100, 100, 600, 400, "callback");
    window->begin();

    Fl_Value_Input *vinput = new Fl_Value_Input(40, 40, 40, 40);
    vinput->textsize(30);

    Fl_Button *button = new Fl_Button(160, 40, 120, 40, "Set slider");
    button->labelsize(15);

    Fl_Slider *slid = new Fl_Slider(40, 200, 400, 30);
    slid->slider(FL_UP_FRAME);
    ((Fl_Widget*)slid)->type(FL_HOR_NICE_SLIDER);
    slid->bounds(10, -10);
    slid->slider_size(10);
    slid->range(-10, 10);
    slid->step(1);
    slid->value(0);

    button->callback(set_slider);

    window->end();
    window->show();
    return Fl::run();

}

Answer 1

You are setting your slider in the button callback - the widget is the button: not the slider. Also the vinput you are referring to is a global, not the local in main.

To fix, pass a structure with both the input and the slider to the callback.

// Create a structure for input and slider
struct Info
{
    FL_Value_Input* vinput;
    FL_Slider* slider;
}

// Callback
void set_slider(FL_Widget* w, void* v)
{
    Info* info = (Info*) v;
    info->slider->value(info->vinput->value());
}
....
// Declare Info in your main code
Info info;

...
info->vinput = new Fl_value_input...
...
info->slider = new Fl_slider...
...
button->callback(set_slider, (void*)&info);