Question

Can macro return an object?

#define macro1 {obj1}

Since macro is text substitution, could I use macro like macro1.function1()?

Thanks.

Answer 1

A macro never returns anything. A macro returns textual representation of code that will be pasted into the place of the program where it is used before compilation.

Read about the C Preprocessor.

So:

#define macro1 {obj1}

int main() {
  macro1
}

… will be compiled as if you wrote

int main() {
  {obj1}
}

It’s just textual substitution, that can optionally take a parameter.

---

If you’re using GCC you can take a look at what the program will look like after preprocessing, using the cpp tool:

# cpp myprog.cpp myprog.cpp_out

Generally mixing macro’s with objects is bad practice, use templates instead.

---

One known usage of macro’s in terms of objects is using them to access singletons (however that isn’t such a nice idea in general):

#define LOGGER Logger::getLogger()

...

LOGGER->log("blah");

You could also use the preprocessor to choose at compile time the object you want to use:

#ifdef DEBUG
#  define LOGGER DebugLogger
#else
#  define LOGGER Logger
#end

// assuming that DebugLogger and Logger are objects not types
LOGGER.log("blah");

… but the forementioned templates do that better.