Question

The following code compiles fine.

#define CMD_MACRO(pp, cmd)  \
{           \
      if (pp)\
      { cmd; }        \
}

template<class T> void operate_on(T &data, char c) {
  data=data+1;
};

int main() {
  int book=4;
  char c;
    CMD_MACRO(book, {
        operate_on<int>(book, c);
    });
};

Note that the actual macro in my code is more complex, I have given a simplified version which may not make much logical sense

Now, if I add another template parameter in the function it gives compilation error (problem explained in code comment):

template<class T, bool b> void operate_on(T &data, char c) {
  data=data+1;
};
int main() {
      int book=4;
      char c;
        CMD_MACRO(book, {
            operate_on<int, false>(book, c); /* here the "," between int and 
                  false is being treated 
                  as separating arguments to CMD_MACRO, 
                  instead being part of 'cmd'. Thats strange 
                  because the comma separating book and c is 
                  treated fine as part of 'cmd'. */
        });
};


test.cpp:18:6: error: macro "CMD_MACRO" passed 3 arguments, but takes just 2
test.cpp: In function 'int main()':
test.cpp:16: error: 'CMD_MACRO' was not declared in this scope

How to fix the problem (I need to add that extra template parameter to existing code and am getting such an error).

Answer 1

Have you tried: (operate_on<int, false>(book, c));? (Notice the extra parentheses around the expression).

I believe the preprocessor knows nothing of C++ templates, and so treats the < and > as just any old token. Without the extra parentheses, it treats operate_on<int as one argument, and false>(book, c) as another.