What should be done with macros that need to paste two tokens together?, C++ FAQ

C++ FAQ / Section 39 / FAQ 39.6

Section 39:

39.1	How do I convert a value (a number, for example) to a `std::string`?
39.2	How do I convert a `std::string` to a number?
39.3	Can I templatize the above functions so they work with other types?
39.4	What should be done with macros that contain `if`?
39.5	What should be done with macros that have multiple lines?
39.6	What should be done with macros that need to paste two tokens together? Updated!
39.7	Why can't the compiler find my header file in `#include "c:\test.h"` ?
39.8	What are the C++ scoping rules for `for` loops?
39.9	Why can't I overload a function by its return type?
39.10	What is "persistence"? What is a "persistent object"?
39.11	How can I create two classes that both know about each other?
39.12	What special considerations are needed when forward declarations are used with member objects?
39.13	What special considerations are needed when forward declarations are used with inline functions?
39.14	Why can't I put a forward-declared class in a `std::vector<>`?
39.15	Why do some people think `x = ++y + y++` is bad?
39.16	What's the deal with "sequence points"?

[39.6] What should be done with macros that need to paste two tokens together? Updated!

[Recently capitalized macro name3() to NAME3() to prevent name-collisions. Click here to go back to the beginning of the "chain" of recent changes]

Groan. I really hate macros. Yes they're useful sometimes, and yes I use them. But I always wash my hands afterwards. Twice. Macros are evil in 4 different ways: evil#1, evil#2, evil#3, and evil#4.

Okay, here we go again, desperately trying to make an inherently evil thing a little less evil.

First, the basic approach is use the ISO/ANSI C and ISO/ANSI C++ "token pasting" feature: ##. On the surface this would look like the following:

Suppose you have a macro called "MYMACRO", and suppose you're passing a token as the parameter of that macro, and suppose you want to concatenate that token with the token "Tmp" to create a variable name. For example, the use of MYMACRO(Foo) would create a variable named FooTmp and the use of MYMACRO(Bar) would create a variable named BarTmp. In this case the naive approach would be to say this:

#define MYMACRO(a) \
    /*...*/ a ## Tmp /*...*/

However you need a double layer of indirection when you use ##. Basically you need to create a special macro for "token pasting" such as:

#define NAME2(a,b)         NAME2_HIDDEN(a,b)
#define NAME2_HIDDEN(a,b)  a ## b

Trust me on this — you really need to do this! (And please nobody write me saying it sometimes works without the second layer of indirection. Try concatenating a symbol with __LINE__ and see what happens then.)

Then replace your use of a ## Tmp with NAME2(a,Tmp):

#define MYMACRO(a) \
    /*...*/ NAME2(a,Tmp) /*...*/

And if you have a three-way concatenation to do (e.g., to paste three tokens together), you'd create a NAME3() macro like this:

#define NAME3(a,b,c)         NAME3_HIDDEN(a,b,c)
#define NAME3_HIDDEN(a,b,c)  a ## b ## c