Simplifying the Makefile - c++

Question

I use this Makefile to build a small C++ application:
BIN_CPP=Main
CPP=g++
INCLUDES_APR=/usr/local/apr/include/apr-1
LIB_SRC = $(wildcard My*.cpp)
LIB_OBJ = $(LIB_SRC:.cpp=.o)
RM=rm
all: Main
MyClass.o: MyClass.cpp
$(CPP) -I$(INCLUDES_APR) -c $< -o $#
MyModel.o: MyModel.cpp
$(CPP) -I$(INCLUDES_APR) -c $< -o $#
libMyLibrary.so: $(LIB_OBJ)
$(CPP) -fPIC -shared -o $# $^
Main.o: Main.cpp
$(CPP) -o $# -c $^ -I$(INCLUDES_APR)
Main: libMyLibrary.so Main.o
$(CPP) $^ -o $# -L/usr/local/apr/lib -L. -lapr-1 -lMyLibrary
.PHONY: clean
clean:
$(RM) -f *.o *.so $(BIN_CPP)
When I remove then first two targets and extend the libMyLibrary.so one, it fails:
# MyClass.o: MyClass.cpp
# $(CPP) -I$(INCLUDES_APR) -c $< -o $#
# MyModel.o: MyModel.cpp
# $(CPP) -I$(INCLUDES_APR) -c $< -o $#
libMyLibrary.so: $(LIB_OBJ)
$(CPP) -fPIC -shared -o $# $^ -I$(INCLUDES_APR)
and the error message is this:
g++ -c -o MyClass.o MyClass.cpp
In file included from MyClass.cpp:1:
MyClass.hpp:3:10: fatal error: apr_general.h: No such file or directory
3 | #include <apr_general.h>
| ^~~~~~~~~~~~~~~
compilation terminated.
make: *** [<builtin>: MyClass.o] Error 1
The -I$(INCLUDES_APR) is missing from the automake output. What is wrong with this build file?

Answer 1

By removing the explicit rules, you are relying on GNU make's built-in rules to compile your files, which is good. But GNU make's built-in rules can't possibly know about your local variable INCLUDES_APR, so when it compiles the source files that variable is not used.
You should add the -I flag to the standard variable CPPFLAGS (the "CPP" here stands for "C preprocessor", not "C++"), which is what make uses to compile in its built-in rules.
Example:
BIN_CPP=Main
CPP=g++
INCLUDES_APR=/usr/local/apr/include/apr-1
CPPFLAGS=-I$(INCLUDES_APR)
LIB_SRC = $(wildcard My*.cpp)
LIB_OBJ = $(LIB_SRC:.cpp=.o)
RM=rm
all: Main
libMyLibrary.so: $(LIB_OBJ)
$(CPP) -fPIC -shared -o $# $^
Main: Main.o libMyLibrary.so
$(CPP) $< -o $# -L/usr/local/apr/lib -L. -lapr-1 -lMyLibrary
.PHONY: clean
clean:
$(RM) -f *.o *.so $(BIN_CPP)
Possible make output
g++ -I/usr/local/apr/include/apr-1 -c -o Main.o Main.cpp
g++ -I/usr/local/apr/include/apr-1 -c -o MyClass.o MyClass.cpp
g++ -I/usr/local/apr/include/apr-1 -c -o MyModel.o MyModel.cpp
g++ -fPIC -shared -o libMyLibrary.so MyClass.o MyModel.o
g++ Main.o -o Main -L/usr/local/apr/lib -L. -lapr-1 -lMyLibrary