Makefile compiles all files, even though changes are done in a single c++ file -

i work on project involving large number of c++ files. asking question out of curiosity.

usually after make done, particular c++ file under modification compiled alone subsequent makes. experience small modification in single c++ file results compilation of other c++ files , final executable.

what reason makefile compile other non-modified c++ files?

so here comes concept of timestamping in makefile. example make rule this

output: dep1      rule-1 dep1: dep2      rule-2 

when execute makefile make first checks timestamp of output file, (if output file doesn't exists default rule executed). output created @ 1200hrs , change file dep1 @ 1205hrs , execute make, make finds target output older depepndency dep1 execute rule-1. if suppose output created @ 1300hrs, dep1 @ 1230hrs, , update dep2 @ 1310hrs, , execute make. can see here since dep1 older dep2 rule-2 executed , dep1's timestamp has been changed, output older dep1 rule-1 executed. can see here though output doesn't directly depend on dep2 rebuilt when updated dep2

this happening in case. check makefile , try check dependency of file edited. targets depend on edited file rebuilt.