21_eval_dynamic_rules

Lesson 21 — $(eval) and Dynamic Rules

What you'll learn

How to use $(eval), $(call), and $(foreach) to generate Makefile rules from a template, eliminating repetitive boilerplate when building multiple similar targets.

The problem

In lesson 19, each executable required its own block of variables and a hand-written link rule:

TEMP_SRCS = src/temp_tool.cpp
TEMP_OBJS = $(patsubst %.cpp,$(BUILDDIR)/%.o,$(TEMP_SRCS))
TEMP_BIN  = $(BUILDDIR)/temp_tool

$(TEMP_BIN): $(TEMP_OBJS) $(LIB)
	@mkdir -p $(@D)
	$(CXX) -o $@ $(TEMP_OBJS) $(LIB)

Two executables means two nearly-identical blocks. Three means three. Ten means ten. Every new program is copy-paste with a find-and-replace on the name, which is tedious and error-prone.

The solution: $(eval) + $(call) + $(foreach)

The idea is to write the boilerplate once as a template, then stamp it out for each program in a list.

1. Define the template

define make_program
$(1)_SRCS = src/$(1).cpp
$(1)_OBJS = $$(patsubst %.cpp,$$(BUILDDIR)/%.o,$$($(1)_SRCS))
$(1)_BIN  = $$(BUILDDIR)/$(1)

$$($(1)_BIN): $$($(1)_OBJS) $$(LIB)
	@mkdir -p $$(@D)
	$$(CXX) -o $$@ $$($(1)_OBJS) $$(LIB)
endef

$(1) is the first argument passed to $(call). Everything else follows the $$ escaping rules described below.

2. Stamp out the rules

PROGRAMS = server client admin
$(foreach prog,$(PROGRAMS),$(eval $(call make_program,$(prog))))

This single line replaces all three hand-written blocks. To add a fourth program, just append its name to PROGRAMS and create src/name.cpp.

3. Collect derived variables

ALL_BINS = $(foreach prog,$(PROGRAMS),$($(prog)_BIN))
ALL_OBJS = $(LIB_OBJS) $(foreach prog,$(PROGRAMS),$($(prog)_OBJS))

$($(prog)_BIN) is a double expansion: first $(prog) expands to server, then $(server_BIN) expands to the actual path. This works because $(eval) already defined those variables.

How $$ escaping works

The $(call) + $(eval) pipeline processes the template in two passes:

Pass	When	What $ does
First pass ($(call) + $(eval))	Makefile parse time	Single $ is expanded. $(1) becomes the argument. $$ becomes a literal $.
Second pass (Make's normal evaluation)	Rule/recipe execution time	The surviving $ expressions are expanded as usual.

Concrete example inside the template:

You write	After first pass (eval time)	After second pass (recipe time)
$(1)	server	(already resolved)
$$(CXX)	$(CXX)	g++
$$(@D)	$(@D)	build
$$($(1)_OBJS)	$(server_OBJS)	build/src/server.o

The rule of thumb: use single $ for things that should resolve during eval (template arguments like $(1)), and double $$ for things that should resolve later (Make variables, automatic variables, and function calls).

Watch out: default goal

$(eval) creates rules at parse time. If the first $(eval)-generated rule appears before your all: target, Make will treat it as the default goal and only build that one program. Fix this by setting .DEFAULT_GOAL = all before the $(foreach) line.

Debugging eval'd rules

To see what the template expands to before $(eval) interprets it, temporarily replace $(eval ...) with $(info ...):

$(foreach prog,$(PROGRAMS),$(info $(call make_program,$(prog))))

This prints the generated Makefile text to stdout without actually defining any rules. You can verify that the variable names, prerequisites, and recipes look correct. Once satisfied, switch back to $(eval ...).

Try it

cd 21_eval_dynamic_rules
make                    # builds all three programs
./build/server          # [server] Running MyApp v1.0
./build/client          # [client] Connected to MyApp v1.0
./build/admin           # [admin] Managing MyApp v1.0
make clean

To add a new program, just add its name to PROGRAMS and create src/name.cpp. No Makefile rules to copy.

Files

File	Purpose
include/config.hpp	Declares get_app_name()
src/config.cpp	Implements get_app_name(), shared by all programs
src/server.cpp	Server executable
src/client.cpp	Client executable
src/admin.cpp	Admin executable
Makefile	Demonstrates $(eval) + $(call) + $(foreach) to generate per-program rules