makefile - Could someone explain this make file? -

i found makefile on site. don't explain example, wondering if new going on.

cc=g++ cflags=-c -wall ldflags= sources=main.cpp hello.cpp factorial.cpp objects=$(sources:.cpp=.o) executable=hello  all: $(sources) $(executable)  $(executable): $(objects)      $(cc) $(ldflags) $(objects) -o $@  .cpp.o:     $(cc) $(cflags) $< -o $@ 

cc=g++ cflags=-c -wall ldflags= sources=main.cpp hello.cpp factorial.cpp 

sets 4 variables constant strings. rest of makefile, wherever $(cc) appears (for example), replaced g++

objects=$(sources:.cpp=.o) 

sets variable objects same sources, except wherever pattern .cpp appears in words of sources, replaced .o

executable=hello 

sets constant string var

all: $(sources) $(executable) 

the first actual rule in makefile, tells make build all must first build in $(sources) , $(executable), , nothing. since first, becomes default target, running make equivalent make all

$(executable): $(objects)          $(cc) $(ldflags) $(objects) -o $@ 

another rule: create $(executable) (which expands hello) must first build in $(objects) (equivalent main.o hello.o factorial.o) , run command $(cc) $(ldflags) $(objects) -o $@

.cpp.o:         $(cc) $(cflags) -o $@ $< 

a pattern rule: in order build file ending in .o, first rebuild/create/find corresponding file ending in .cpp, , run command $(cc) $(cflags) -o $@ $<.

these last 2 rules contain special variables $@ , $< valid in rule actions , expand target , first dependency respectively

so when run make, reads , tries build default target (all). since doesn't exist, tries build files main.cpp, hello.cpp, factorial.cpp, , hello. since first 3 (presumably) exist, looks rules/dependencies them, doesn't find any, decides there's nothing them. if didn't exist, make give error saying "no rule make target 'main.cpp'"

in case of "hello" depends on main.o, hello.o , factorial.o, looks them. main.o, pattern rule says depends on main.cpp, if main.o doesn't exist or if main.cpp newer, run command g++ -c -wall -o main.o main.cpp. same happens hello.o , factorial.o.

once done, if hello doesn't exist or older of .o files (which may have changed, possibly pretty new), run command relink it. finally, run empty command (doing nothing) 'rebuild' all.