For Qt qmake variables, I sometimes see different notation.
Examples
$$PWD $${PWD}
To me it seems the same, what's the difference?
According to the documentation, the two entries are almost equivalent. The only difference is that the second notation allows you to append the contents of the variable to another value without separating the two with a space.
the second notation allows you to append the contents of the variable to another value without separating the two with a space