{"id":103559,"date":"2020-03-13T07:00:00","date_gmt":"2020-03-13T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103559"},"modified":"2020-03-12T21:47:00","modified_gmt":"2020-03-13T04:47:00","slug":"20200313-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200313-00\/?p=103559","title":{"rendered":"Providing a better error message when someone tries to use std::vector<bool><\/CODE> as a buffer"},"content":{"rendered":"

Last time<\/a>, we looked at how we could generate a useful error message if somebody tried to pass std::vector<bool><\/code> to our buffer_<\/code>view<\/code> class. The std::vector<bool><\/code> is unlike all the other std::vector<T><\/code> types because it does not require its storage to be in the form of a traditional C array, which means that it is not possible to obtain direct access to the underlying storage.<\/p>\n

Last time, we struggled with this buffer type:<\/p>\n

struct buffer_view\r\n{\r\n  template<typename C>\r\n  buffer_view(std::vector<C> const& v) :\r\n    data(v.data()), size(v.size() * sizeof(C)) { }\r\n\r\n  \/\/ Imagine other constructors for std::array, etc.\r\n\r\n  void const* data;\r\n  std::size_t size;\r\n};\r\n<\/pre>\n
If somebody tries to create a buffer_<\/code>view<\/code> from a std::vector<bool><\/code>, they get an incomprehensible error message because there is no v.data()<\/code> method. (For some reason, gcc and clang do have a data()<\/code> method, but it doesn’t return anything interesting, so the error message is even more<\/i> incomprehensible.)<\/p>\n
We addressed the problem last time by introducing an overload of the constructor that is active only for std::vector<bool><\/code>, and putting a static_<\/code>assert<\/code> in the body with a deceptively type-dependent expression so that the assertion wasn’t raised until the overload was invoked.<\/p>\n
I noted that  Kenny Kerr<\/a> came up with a simpler solution:  Move the call to data()<\/code> to a helper function<\/a>, and templatize that helper.<\/p>\n
struct buffer_view\r\n{\r\n  template<typename C>\r\n  buffer_view(std::vector<C> const& v) :\r\n    data(get_data(v)<\/span>), size(v.size() * sizeof(C)) { }\r\n\r\n  \/\/ Imagine other constructors for std::array, etc.\r\n\r\n  void const* data;\r\n  std::size_t size;\r\n\r\nprivate:\r\n  void const* get_data(std::vector<C> const& v)\r\n  {\r\n    static_assert(!is_same_v<C, bool>,\r\n      \"Can't use std::vector<bool>. Try std::array instead.\");\r\n    return v.data();\r\n  }<\/span>\r\n};\r\n<\/pre>\n
The static_<\/code>assert<\/code> comes ahead of the call to v.data()<\/code>, so it becomes the first error message.<\/p>\n
You could go even further and make it the only<\/i> error message by adding some if constexpr<\/code>:<\/p>\n
  void const* get_data(std::vector<C> const& v)\r\n  {\r\n    static_assert(!is_same_v<C, bool>,\r\n      \"Can't use std::vector<bool>. Try std::array instead.\");\r\n    if constexpr (!is_same_v<C, bool>) {\r\n      return v.data();\r\n    } else {\r\n      return nullptr;\r\n    }<\/span>\r\n  }\r\n<\/pre>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
Defer the problem further.<\/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-103559","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Defer the problem further.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103559","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=103559"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103559\/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=103559"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103559"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103559"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}