{"id":5013,"date":"2008-04-30T21:15:00","date_gmt":"2008-04-30T21:15:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vcblog\/2008\/04\/30\/enums-macros-unicode-and-token-pasting\/"},"modified":"2019-02-18T18:54:07","modified_gmt":"2019-02-18T18:54:07","slug":"enums-macros-unicode-and-token-pasting","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/cppblog\/enums-macros-unicode-and-token-pasting\/","title":{"rendered":"Enums, Macros, Unicode and Token-Pasting"},"content":{"rendered":"

Enums, Macros, Unicode and Token-Pasting<\/font><\/p>\n

Hi, I am Rocky a developer on the Visual C++ IDE team.  <\/span>I would like to discuss the C++ programming technique of creating macro generated enums.  <\/span>I recently used this for distinguishing various C++ types such as class, variable, and function.  <\/span>Having the list of types in one file makes it easy to add types and allows for many different types of uses.  <\/span>The examples I have below mirror uses in code that I have worked with.  <\/span>I have also seen this technique used in many places in various other source bases, but I have not seen it discussed much in text books – so I thought I would highlight this technique.<\/font><\/p>\n

Consider this enum:<\/font><\/p>\n

enum<\/span> Animal { dog, cat, fish, bird };<\/p>\n

<\/span><\/p>\n

Now dog can be used in place of 0.  <\/span>You can get compiler enforced type safety that macros do not provide.  <\/span>The VS debugger will also show the friendly value of the enum instead of integers.  <\/span>However, functions that print out enum values need better formatting.  <\/span>This code can help:<\/font><\/p>\n

wchar_t<\/span>* AnimalDiscription[] = { L“dog”<\/span>, L“cat”<\/span>, L“fish”<\/span>, L“bird”<\/span> };<\/span><\/p>\n

With this array, debugging code can now print the friendly value of the enum by using the enum to index into the string array.<\/font><\/p>\n

With macro generated enums both the enum and the friendly names can be maintained as one entity.  <\/span>Consider the file animal.inc:<\/font><\/p>\n

MYENUM(dog)<\/p>\n

<\/span><\/p>\n

MYENUM(cat)<\/p>\n

<\/span><\/p>\n

MYENUM(fish)<\/p>\n

<\/span><\/p>\n

MYENUM(bird)<\/span><\/p>\n

And the following C++ code:<\/font><\/p>\n

enum<\/span> Animal {<\/p>\n

<\/span><\/p>\n

#define<\/span> MYENUM(e) _##e,<\/p>\n

<\/span><\/p>\n

#include<\/span> “animal.inc”<\/p>\n

<\/span><\/span><\/p>\n

#undef<\/span> MYENUM<\/p>\n

<\/span><\/p>\n

};<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

wchar_t<\/span>* AnimalDescription[] = {<\/p>\n

<\/span><\/p>\n

#define<\/span> MYENUM(e) L“_”<\/span> L#e,<\/p>\n

<\/span><\/p>\n

#include<\/span> “animal.inc”<\/p>\n

<\/span><\/span><\/p>\n

#undef<\/span> MYENUM<\/p>\n

<\/span><\/p>\n

};<\/p>\n

<\/span><\/p>\n

<\/font> <\/p>\n

Now editing animal.inc will update both the enum and the friendly text of the enum.  <\/span>In this case I added an underscore in front of the animal names I used before to get the macro to work correctly.  <\/span>The token-pasting operator ## cannot be used as the first token.  <\/span>The stringizing operator # creates a string from the operator.  <\/span>By adding an L right before the stringizing opera\ntor the resulting string is a wide string.<\/font><\/p>\n

These macro generated enums can be “debugged” by using the compiler switch \/EP or \/P.  <\/span>This will cause the compiler to output the preprocessor file:<\/font><\/p>\n

enum<\/span> Animal {<\/p>\n

<\/span><\/p>\n

_dog,<\/p>\n

<\/span><\/p>\n

_cat,<\/p>\n

<\/span><\/p>\n

_fish,<\/p>\n

<\/span><\/p>\n

_bird,<\/p>\n

<\/span><\/p>\n

};<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

wchar_t<\/span>* AnimalDescription[] = {<\/p>\n

<\/span><\/p>\n

L“_”<\/span> L“dog”<\/span>,<\/p>\n

<\/span><\/p>\n

L“_”<\/span> L“cat”<\/span>,<\/p>\n

<\/span><\/p>\n

L“_”<\/span> L“fish”<\/span>,<\/p>\n

<\/span><\/p>\n

L“_”<\/span> L“bird”<\/span>,<\/p>\n

<\/span><\/p>\n

};<\/span><\/p>\n

C++ allows for a comma after the last entry of the enum and the array initializer.<\/font><\/p>\n

This macro string replacement technique can be further expanded to produce code.  <\/span>Here is an example of using string replacement to create function prototypes:<\/font><\/p>\n

#define<\/span> MYENUM(e) void<\/span> Order_##e();<\/p>\n

<\/span><\/p>\n

#include<\/span> “animal.inc”<\/p>\n

<\/span><\/span><\/p>\n

#undef<\/span> MYENUM<\/p>\n

<\/span><\/p>\n

This expands to:<\/p>\n

<\/span><\/font><\/font><\/p>\n

void<\/span> Order_dog();<\/p>\n

<\/span><\/p>\n

void<\/span> Order_cat();<\/p>\n

<\/span><\/p>\n

void<\/span> Order_fish();<\/p>\n

<\/span><\/p>\n

void<\/span> Order_bird();<\/p>\n

<\/span><\/p>\n

You may wish to do some action based on the kind of animal.  <\/span>If you switch on the kind of animal, here is an example of creating case labels and function calls:<\/font><\/p>\n

#define<\/span> MYENUM(e) case<\/span> _##e:<\/p>\n

<\/span><\/p>\n

                   <\/span><\/span>Order_##e();<\/p>\n

<\/span><\/p>\n

                   <\/span>break<\/span>;<\/p>\n

<\/span><\/p>\n

#include<\/span> “animal.inc”<\/p>\n

<\/span><\/span><\/p>\n

#undef<\/span> MYENUM<\/p>\n

<\/span><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

This expands to:<\/font><\/p>\n

case<\/span> _dog: Order_dog(); break<\/span>;<\/p>\n

<\/span><\/p>\n

case<\/span> _cat: Order_cat(); break<\/span>;<\/p>\n

<\/span><\/p>\n

case<\/span> _fish: Order_fish(); break<\/span>;<\/p>\n

<\/span><\/p>\n

case<\/span> _bird: Order_bird(); break<\/span>;<\/p>\n

<\/span><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

In this example the function definitions would need to be added for each of the Order_dog(), Order_cat(), etc..  <\/span>If you were to add a new animal to animal.inc, you would not need to remember that you would also need to add a new Order_ function definition for this new animal.  <\/span>The linker would give you an error reminding you!<\/font><\/p>\n

Macro string replacement is a powerful tool that can be leveraged to allow for internal data to be stored in one spot.  <\/span>Keeping this data in one spot reduces the chances for errors, missed cases or missed matched cases.<\/span><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Enums, Macros, Unicode and Token-Pasting Hi, I am Rocky a developer on the Visual C++ IDE team.  I would like to discuss the C++ programming technique of creating macro generated enums.  I recently used this for distinguishing various C++ types such as class, variable, and function.  Having the list of types in one file makes […]<\/p>\n","protected":false},"author":289,"featured_media":35994,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-5013","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-cplusplus"],"acf":[],"blog_post_summary":"

Enums, Macros, Unicode and Token-Pasting Hi, I am Rocky a developer on the Visual C++ IDE team.  I would like to discuss the C++ programming technique of creating macro generated enums.  I recently used this for distinguishing various C++ types such as class, variable, and function.  Having the list of types in one file makes […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/posts\/5013","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/users\/289"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/comments?post=5013"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/posts\/5013\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/media\/35994"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/media?parent=5013"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/categories?post=5013"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/cppblog\/wp-json\/wp\/v2\/tags?post=5013"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}