Undefined environment variable and operator ?=

[zosrothko]

Hello

Here a simple makefile that sets the variable CXX if not defined to
$(FOO)g++

CXX     ?=$(FOO)g++

test:
        @echo $(CXX)

and the some stories

address@hidden:~/make$ unset CXX
address@hidden:~/make$ make
g++
address@hidden:~/make$ make FOO=bar
g++
address@hidden:~/make$ FOO=bar make
g++
address@hidden:~/make$ export FOO=bar;make
g++

Why CXX is never set to 'barg++' ?