{"id":86,"date":"2018-12-25T02:49:23","date_gmt":"2018-12-25T05:49:23","guid":{"rendered":"http:\/\/35.243.195.209\/?p=86"},"modified":"2019-01-25T23:18:13","modified_gmt":"2019-01-26T07:18:13","slug":"auto-type","status":"publish","type":"post","link":"https:\/\/nanzhou.cc\/index.php\/2018\/12\/25\/auto-type\/","title":{"rendered":"Auto Type"},"content":{"rendered":"

Summary<\/h2>\n

In this post, I will introduce auto<\/code>, const auto<\/code>, auto&<\/code>, const auto&<\/code>, auto&&<\/code>, and const auto&&<\/code>.<\/p>\n

Conclusion<\/h2>\n

We provide the conclusion first.<\/p>\n

    \n
  1. When you want a read-only<\/strong> access (generic or non-generic codes), use auto const &<\/code>, which will give you a reference rather than a copy. <\/li>\n
  2. When you want a read-write<\/strong> access in generic<\/strong> codes, use auto &&<\/code>.<\/li>\n
  3. When you want a read-write<\/strong> access in non-generic<\/strong> codes, use auto &<\/code>.<\/li>\n<\/ol>\n

    Details<\/h2>\n

    auto<\/code><\/h3>\n

    Auto will create a copy of each element in the range so use this variant when you want to work with a copy.<\/p>\n

    Note the following situations.
    \n1. Containers return proxy objects upon dereferencing of their iterators.<\/p>\n

    std::vector<bool> vec{false, false, false};\nfor (auto x : vec) {\n    x = true; \/\/ now vec is {true, true, true}\n}\n<\/code><\/pre>\n
    const auto<\/code><\/h3>\n
    It will provide an immutable copy of each element.
    \nHowever we should avoid using const auto<\/code> because there is no reason we create a copy. Thus, use const auto &<\/code> instead.<\/p>\n
    const auto&<\/code> or auto const &<\/code><\/h3>\n
    This is the number one choice<\/strong> for iterating over a range when all you want to is read its elements. No copies are made and the compiler can verify that you indeed do not modify the elements.<\/p>\n
    auto&<\/code><\/h3>\n
    Use auto&<\/code> when you want to modify elements in the range in non-generic codes. &<\/code> means it will make references to the original elements in the range. See special situations (eg, inside templates) in reference<\/a>.<\/p>\n
    auto&&<\/code><\/h3>\n
    Use auto&&<\/code> when you want to modify elements in the range in generic codes. Actually it also works in non-generic codes, but it unnecessarily confuses people. Details are in reference<\/a>.<\/p>\n
    const auto&&<\/code><\/h3>\n
    There is no reason for choosing this variant over const auto&<\/code>.<\/p>\n
    Reference<\/h2>\n
    See this blog<\/a> for more details and some special situations.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Summary In this post, I will introduce auto, const auto, auto&, const auto&, auto&&, and const auto&&. Conclusion We provide the conclusion first. When you want a read-only access (generic or non-generic codes), use auto const &, which will give you a reference rather than a copy. When you want a read-write access in generic…<\/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-86","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\/86","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=86"}],"version-history":[{"count":5,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/86\/revisions"}],"predecessor-version":[{"id":227,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/86\/revisions\/227"}],"wp:attachment":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/media?parent=86"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/categories?post=86"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/tags?post=86"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}