{"id":600,"date":"2019-09-26T23:24:24","date_gmt":"2019-09-27T06:24:24","guid":{"rendered":"http:\/\/35.243.195.209\/?p=600"},"modified":"2019-09-26T23:28:39","modified_gmt":"2019-09-27T06:28:39","slug":"in-class-initialization-errors","status":"publish","type":"post","link":"https:\/\/nanzhou.cc\/index.php\/2019\/09\/26\/in-class-initialization-errors\/","title":{"rendered":"In-Class Initialization Errors"},"content":{"rendered":"

Summary<\/h2>\n

In this post, I will introduce some common compiling errors I used to see related to in-class initialization: <\/p>\n

    \n
  1. Error: in-class initialization of static data member xxx of non-literal type;<\/li>\n
  2. Error: ISO C++ forbids in-class initialization of non-const static member xxx;<\/li>\n<\/ol>\n

    Conclusion<\/h2>\n

    As usual, I present the correct ways to do in-class initialization at the very beginning. <\/p>\n

    \/\/ header.h\nclass Base {\npublic:\n    static int count;\n\n    static char const static_char = 'a';\n    static int const static_int = 1;\n\n    static constexpr std::pair<int, int> static_constexpr_pair = {0, 1};\n\n    static std::string static_var;\n    static std::string const static_const_var;\n    std::string const const_var = "const_var";\n\n    std::vector<int> const const_vec = {0, 1, 2};\n\n    Base() {\n        count++;\n    }\n};\n\n\/\/ src.cpp\nint Base::count = 0;\n\nstd::string Base::static_var = "static_var";\nstd::string const Base::static_const_var = "static_const_var";<\/code><\/pre>\n
    Some golden rules we can follow are:<\/p>\n
      \n
    1. For static varibles, do not initialize them Inline;<\/li>\n
    2. Always initialize static varibles in the source file;<\/li>\n
    3. You can do any kind of inline initialization for non-static variable. <\/li>\n<\/ol>\n
      Details<\/h2>\n
      Why I can’t initialize static data members in class?<\/h3>\n
      You will get "Error: in-class initialization of static data member xxx of non-literal type" in the following codes. <\/p>\n
      class Base {\npublic:\n    static std::string static_var = "static_var";\n    static std::string const static_const_var = "static_const_var";\n}<\/code><\/pre>\n
      Bjarne explains this aptly here<\/a>:<\/p>\n
      "A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects."<\/p>\n
      Inline initialization of static member variables<\/h3>\n
      There are a few shortcuts to the above. First, when the static member is a const integral type (which includes char and bool) or a const enum, the static member can be initialized inside the class definition:<\/p>\n
      \/\/ header.h\nclass Base {\npublic:\n    static char const static_char = 'a';\n    static int const static_int = 1;\n}<\/code><\/pre>\n
      The reason is "Static const integers (and enums) can be treated as compile-time constants, so the compiler can replace calls to the const integer with the value itself."<\/p>\n
      Second, as of C++11, static constexpr members of any type that supports constexpr initialization can be initialized inside the class definition:<\/p>\n
      class Base {\npublic:\n    static constexpr std::pair<int, int> static_constexpr_pair = {0, 1};\n}<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
      Summary In this post, I will introduce some common compiling errors I used to see related to in-class initialization: Error: in-class initialization of static data member xxx of non-literal type; Error: ISO C++ forbids in-class initialization of non-const static member xxx; Conclusion As usual, I present the correct ways to do in-class initialization at the…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,5],"tags":[],"class_list":["post-600","post","type-post","status-publish","format-standard","hentry","category-c","category-proglang"],"_links":{"self":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/600","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/comments?post=600"}],"version-history":[{"count":4,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/600\/revisions"}],"predecessor-version":[{"id":604,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/600\/revisions\/604"}],"wp:attachment":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/media?parent=600"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/categories?post=600"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/tags?post=600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}