Single Makefile to build both main.c and main.cpp - c++

Question

I would like to make main-c to build main-c from main.c.
And make main-cpp to build main-cpp from main.cpp.
I have, all in the same folder:
main.c:
#include <stdio.h>
int main(int argc, char const *argv[]) {
printf("This ic C\n");
}
main.cpp:
#include <iostream>
int main(int argc, char const *argv[]) {
std::cout << "This is C++" << std::endl;
}
Makefile:
CFLAGS = -std=gnu11
CXXFLAGS = -std=gnu++11
CPPFLAGS = -g -Wall -O3
LDFLAGS =
LDLIBS =
OBJS = main.o
APP-C = main-c
APP-CPP = main-cpp
default:
echo "Check README.txt"
main-c: $(OBJS)
$(CC) $^ $(LDLIBS) -o $#
main-cpp: $(OBJS)
$(CXX) $^ $(LDLIBS) -o $#
clean:
-rm -Rf *.o
distclean: clean
-rm -Rf $(APP-C) $(APP-CPP)
So:
$ make main-c
cc -std=gnu11 -g -Wall -O3 -c -o main.o main.c
cc main.o -o main-c
But (also builds from .c):
$ make main-cpp
cc -std=gnu11 -g -Wall -O3 -c -o main.o main.c
c++ main.o -o main-cpp
I was expecting make main-cpp to use $(CXX) $^ $(LDLIBS) -o $# and build from .cpp. What am I missing?

Answer 1

I was expecting make main-cpp to use $(CXX) $^ $(LDLIBS) -o $# and build from .cpp.
It is. make main-cpp --debug will help you see what is going on.
The problem is you are relying on the default rule for making the main.o needed by both main-cpp and main-c target, but you want a different build when make-cpp target is invoked. In this case you'll need to define different non default rules to build the .o files. It's easiest if you just make make-cpp and main-c depend on a .o with a different name. Since they are different builds they should have different names anyway:
CFLAGS = -std=gnu11
CXXFLAGS = -std=gnu++11
CPPFLAGS = -g -Wall -O3
LDFLAGS =
LDLIBS =
APP-C = main-c
APP-CPP = main-cpp
default:
echo "Check README.txt"
main-c: %:%.o
$(CC) $^ $(LDLIBS) -o $#
main-cpp: %:%.o
$(CXX) $^ $(LDLIBS) -o $#
main-cpp.o: main.cpp
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $? -o $#
main-c.o: main.c
$(CC) $(CPPFLAGS) $(CFLAGS) -c $? -o $#
clean:
-rm -Rf *.o
distclean: clean
-rm -Rf $(APP-C) $(APP-CPP)

Answer 2

Both rules, main-c and main-cpp, require main.o to be created to complete, but you don't have a rule for creating main.o.
You're using make's implicit object file creation which is a built-in rule for creating a '.o' file from a source file. It can use either a '.c' or '.cpp/cc/C' file and will build using the appropriate variables. However, since you have both main.c and main.cpp, it looks like it defaults to using the C version.
You'd probably need to create explicit rules (and probably use different names) for each one.