GNU make sometime doesn't search in folders declared in VPATH - c++

Question

I'm trying to build a small C++ project with GNU make (version 3.81) but i must call make two times because the first run fails. This is my project directory:
project
makefile
include
lexer.hpp
src
main.cpp
lexer.l
Following is my makefile:
CC = g++
CPPFLAGS = -I include
VPATH = include src
OBJECTS = main.o lexer.o
test: $(OBJECTS)
$(CC) $(CPPFLAGS) -lfl -o $# $^
main.o: lexer.hpp main.cpp
$(CC) -c $(CPPFLAGS) $^
lexer.o: lexer.cpp
$(CC) -c $(CPPFLAGS) $^
lexer.cpp: lexer.l
flex -t $^ > src/lexer.cpp
.PHONY: clean
clean:
rm -fR $(OBJECTS) src/lexer.cpp test
The first time i run make i get the following output where make complains about not finding the lexer.cpp file. But i don't understand why make don't seek in the folders declared in VPATH.
g++ -c -I include include/lexer.hpp src/main.cpp
flex -t src/lexer.l > src/lexer.cpp
g++ -c -I include lexer.cpp
g++: error: lexer.cpp: No such file or directory
g++: fatal error: no input files
compilation terminated.
make: *** [lexer.o] Error 1
However if i call make again then lexer.cpp is found and the compilation works.
g++ -c -I include src/lexer.cpp
g++ -I include -lfl -o test main.o lexer.o
Why?
P.S. I apologize for poor english.

Answer 1

This rule is wrong:
lexer.cpp: lexer.l
flex -t $^ > src/lexer.cpp
This rule tells make that it will build a file lexer.cpp, and so that's what make is expecting it to do, and after the rule finishes make thinks that file is ready to go, and it will use that filename when other targets depend on it. But what the rule really does, is build src/lexer.cpp.
To write this rule correctly you'll need to write it as:
src/lexer.cpp: lexer.l
flex -t $^ > $#
(every make rule you write should always update the file $#, exactly).
However, in general VPATH is not good for finding generated files (object files, etc.: any file that is generated by make). It's only useful for finding source files (files make doesn't build itself).