Escaping a # symbol in a #define macro?

c++ c macros c-preprocessor preprocessor-directive
Rob picture Rob · Jul 16, 2009 · Viewed 37.2k times · Source

Without going into the gory details I want to use a #define macro that will expand to a #include but the '#' sign is confusing the preprocessor (as it thinks I want to quote an argument.)

For example, I want to do something like this:

#define MACRO(name) #include "name##foo"

And use it thus:

MACRO(Test)

Which will expand to:

#include "Testfoo"

The humble # sign is causing the preprocessor to barf. MinGW gives me the following error:

'#' is not followed by a macro parameter

I guess I need to escape the # sign but I don't if this is even possible.

Yes, macros are indeed evil...

Answer

Yakov Galka picture Yakov Galka · Nov 27, 2011

It is possible to insert a hash token into the preprocessed token stream. You can do it as follows:

#define MACRO(hash, name) hash include name
MACRO(#,"hello")

—expands to:

# include "hello"

However, the standard explicitly rules out any further analysis of such line for the existence of preprocessing directives [cpp.rescan]:

The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one.

Related questions

Can we have recursive macros?

I want to know if we can have recursive macros in C/C++? If yes, please provide a sample example. Second thing: why am I not able to execute the below code? What is the mistake I am doing? Is …

Read More
Comma in C/C++ macro

Say we have a macro like this #define FOO(type,name) type name Which we could use like FOO(int, int_var); But not always as simply as that: FOO(std::map<int, int>, map_var); // error: macro "…

Read More
Creating a string list and an enum list from a C++ macro

In order to make my code shorter and easier to change I want to replace something like enum{ E_AAA, E_BBB, E_CCC }; static const char *strings{"AAA", "BBB", "CCC" }; With a macro, like INIT(AAA, BBB, CCC); but …

Read More