{"id":106002,"date":"2021-12-06T07:00:00","date_gmt":"2021-12-06T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=106002"},"modified":"2021-12-06T06:41:16","modified_gmt":"2021-12-06T14:41:16","slug":"20211206-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20211206-00\/?p=106002","title":{"rendered":"Compiler error message metaprogramming: Helping to find the conflicting macro definition"},"content":{"rendered":"

Say you want to require that a preprocessor macro is set a particular way:<\/p>\n

#include <contoso.h>\r\n#if CONTOSO_VERSION != 314\r\n#error This header file requires version 314.\r\n#endif\r\n<\/pre>\n
Okay, if the version isn’t set correctly, you will indeed get the error, but that doesn’t help the user much with figuring out why the version number is incorrect.<\/p>\n
You might try this:<\/p>\n
#include <contoso.h>\r\n#if CONTOSO_VERSION != 314\r\n#error This header file requires version 314 \\\r\n(got CONTOSO_VERSION instead)\r\n#endif\r\n<\/pre>\n
Unfortunately, it doesn’t work:<\/p>\n
\/\/ MSVC\r\nfatal error C1189: #error:  This header file requires version 314 (got CONTOSO_VERSION instead)\r\n\r\n\/\/ gcc\r\nerror: #error This header file requires version 314 (got CONTOSO_VERSION instead)\r\n\r\n\/\/ clang\r\nerror: This header file requires version 314 (got CONTOSO_VERSION instead)\r\n\r\n\/\/ icc\r\nerror: #error directive: This header file requires version 314 (got CONTOSO_VERSION instead)\r\n<\/pre>\n
None of them substitute the macro in the error message, so you don’t see what version you actually got.<\/p>\n
Here’s the trick: Just redefine the symbol.<\/p>\n
#include <contoso.h>\r\nstatic_assert(CONTOSO_VERSION == 314,\r\n             \"This header file requires version 314.\");\r\n#define CONTOSO_VERSION 314\r\n<\/pre>\n
\/\/ MSVC\r\nerror C2338: This header file requires version 314.\r\nwarning C4005: 'CONTOSO_VERSION': macro redefinition\r\nC:\\contoso\\v271\\contoso.h(5): note: see  previous definition of 'CONTOSO_VERSION'\r\n\r\n\/\/ gcc\r\nwarning: \"CONTOSO_VERSION\" redefined\r\n  3 | #define CONTOSO_VERSION 314\r\n\r\nin file included from widget.h:1:\r\n\/contoso\/v271\/contoso.h:5: note: this is the location of the previous definition\r\n  5 | #define CONTOSO_VERSION 271\r\n\r\nerror: static assertion failed: This header file requires version 314.\r\n  2 | static_assert(CONTOSO_VERSION == 314,\r\n\r\n\/\/ clang\r\nwarning: 'CONTOSO_VERSION' macro redefined [Wmacro-redefined]\r\n#define CONTOSO_VERSION 314\r\n\r\n\/contoso\/v271\/contoso.h:5:9: note: previous definition is here\r\n#define CONTOSO_VERSION 271\r\n\r\nerror: static_assert failed due to requirement '271 == 314' \"This header file requires version 314.\"\r\nstatic_assert(CONTOSO_VERSION == 314,\r\n\r\n\/\/ icc\r\nerror: static assertion failed with \"This header file requires version 314.\"\r\nstatic_assert(CONTOSO_VERSION == 314,\r\n\r\nwarning #47: incompatible redefinition of macro \"CONTOSO_VERSION\" (declared at line 5 of \"\/contoso\/v271\/contoso.h\")\r\n#define CONTOSO_VERSION 314\r\n<\/pre>\n
All of the major compilers provide the courtesy of telling you where the previous conflicting definition was made. From the error message, it is evident that we are including the contoso.h<\/code> header file from the v271<\/code> directory, when we presumably meant to include from the v314<\/code> directory. The thing to investigate, therefore, is the project configuration where the include directories are specified, and fix it so that the correct version of contoso.h<\/code> gets included.<\/p>\n
This trick takes advantage of the fact that the C and C++ languages both permit a macro symbol to be defined twice, provided that the second definition is identical to the first. If not, then the program is ill-formed<\/i> and requires a diagnostic.<\/p>\n
So we just redefine the macro to the value we want. If it has that value, then everything is great. If not, then a diagnostic is required. And all of the major compilers point you at the previous definition, which will help you figure out why there is a conflict.<\/p>\n","protected":false},"excerpt":{"rendered":"
Shaping the code so a required diagnostic will be useful.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-106002","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Shaping the code so a required diagnostic will be useful.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106002","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=106002"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106002\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=106002"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=106002"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=106002"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}