Passing parameter to Makefile to change compiled code

Question

I'm new to make. I am working on a C++ shared library, and I want it to have the option to compile with or without support for a certain feature (block of code). In other words, how do I enable the user to choose whether or not to compile the library with that feature by (maybe) passing a parameter to the make command? For example, I need the user to be able to do this: ``` make --with-feature-x ``` How do I do this? Do I need to write a configure file for example? Or can I do this directly within my Makefile?

Accepted Answer

I believe the following way should work. You define an environment variable when running `make`. In the Makefile, you check the status of the environment variable. Depending on the status, you define the options that will be passed to g++ when compiling the code. g++ Uses the options in the preprocessing phase to decide what to include in the file (e.g. source.cpp).

The command

```
make FEATURE=1
```

Makefile

```
ifeq ($(FEATURE), 1)  #at this point, the makefile checks if FEATURE is enabled
OPTS = -DINCLUDE_FEATURE #variable passed to g++
endif

object:
  g++ $(OPTS) source.cpp -o executable //OPTS may contain -DINCLUDE_FEATURE
```

source.cpp

```
#ifdef INCLUDE_FEATURE 
#include feature.h

//functions that get compiled when feature is enabled
void FeatureFunction1() {
 //blah
}

void FeatureFunction2() {
 //blah
}

#endif
```

To check if FEATURE is passed in or not (as any value):

```
ifdef FEATURE
  #do something based on it
else
  # feature is not defined. Maybe set it to default value
  FEATURE=0
endif
```

Passing parameter to Makefile to change compiled code

Solution

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