c++ - Declaring an array with constant size

can me explain this:

i declare constant in headerfile:

const int incrsize;

then in mil constant initialized:

: incrsize(10)

then later on in function in code (its callback-function used in gtkmm-gui

 bool mywindow::on_drawing_expose_event(gdkeventexpose* event)

i allocate array on stack using constant

double arrpxstep[incrsize];

when compiling following errors:

-expected constant expression - cannot allocate array if constant size 0 - arrpxstep unknown size

i know constant initialzed - how come array cannot use constant memeber-variable alreade initialized in mil?

by looks of code post, incrsize non-static member of class. can't use initialize array, if value set same in constructor. it's rules.

make incrsize static const member, defined within class, , work.

My