Question

I am experimenting with preprocessor function-like macros so I am trying to write a macro that forward declares regular functions. But when I go to compile it says that line 2 has incorrect syntax.

What am I doing wrong?

#define FORWARD_DECLARE_CUSTOM_FUNCT(fName) "int" #fName "(int id, string msg, string cmd);"
FORWARD_DECLARE_CUSTOM_FUNCT("abc") // LINE 2: Should become "int abc(int id, string msg, string cmd);"

void test()
{
    abc(1, "", "");
}

Answer 1

The problem is all of the quotation marks.

#define FORWARD_DECLARE_CUSTOM_FUNCT(fName) "int" #fName "(int id, string msg, string cmd);"

translates to (after concatenating the string literals):

"int abc (int id, string msg, string cmd);"

Whereas,

#define FORWARD_DECLARE_CUSTOM_FUNCT(fName) int #fName (int id, string msg, string cmd);

translates to:

int "abc" (int id, string msg, string cmd);

What you really need is simply:

#define FORWARD_DECLARE_CUSTOM_FUNCT(fName) int fName (int id, string msg, string cmd);

Use it like this:

FORWARD_DECLARE_CUSTOM_FUNCT(abc)

It’s all just text replacement. I would personally remove the semicolon in the macro and make the user put one in after each use as well, to make it more like a statement. It feels more natural to me.