{"id":285,"date":"2019-09-17T23:32:36","date_gmt":"2019-09-17T15:32:36","guid":{"rendered":"http:\/\/www.eolstudy.com\/?p=285"},"modified":"2019-09-18T10:18:47","modified_gmt":"2019-09-18T02:18:47","slug":"c-%e5%a4%9a%e7%ba%a7%e6%8c%87%e9%92%88%e3%80%81%e7%b1%bb%e5%9e%8b%e9%99%90%e5%ae%9a%e7%ac%a6%e4%b8%8e%e7%b1%bb%e5%9e%8b%e8%bd%ac%e6%8d%a2","status":"publish","type":"post","link":"https:\/\/www.eolstudy.com\/index.php\/2019\/09\/study-notes\/cpp\/285\/","title":{"rendered":"C++ \u591a\u7ea7\u6307\u9488\u3001\u7c7b\u578b\u9650\u5b9a\u7b26\u4e0e\u7c7b\u578b\u8f6c\u6362"},"content":{"rendered":"\n<p><em>\u7f16\u8bd1\u5668\uff1a clang 6.0.0-1ubuntu2<\/em><\/p>\n\n\n\n<p>\u81ea\u4f5c\u4e3b\u5f20\u7ffb\u8bd1\u4e86\u4e00\u4e0b\u3002\u4e09\u4e2a\u540d\u8bcd\u5bf9\u5e94\u7684\u539f\u6587\u5206\u522b\u662f multilevel pointer\u3001type qualifier \u4ee5\u53ca type conversion\u3002\u9009\u62e9\u7528\u4e09\u4e2a\u540d\u8bcd\u5e76\u5217\u4f5c\u4e3a\u6807\u9898\uff0c\u4e5f\u5b9e\u5728\u662f\u56e0\u4e3a\u6d89\u53ca\u7684\u5185\u5bb9\u6839\u672c\u65e0\u6cd5\u7528\u7b80\u77ed\u7684\u53e5\u5b50\u63cf\u8ff0\u51fa\u6765\u3002<\/p>\n\n\n\n<p>\u4e0d\u59a8\u7528\u4e00\u4e2a\u4f8b\u5b50\u5f00\u5934\uff1a<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"cpp\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"true\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">int** ppi {nullptr};\nconst int** ppi2 {ppi};<\/pre>\n\n\n\n<p>\u7f16\u8bd1\u62a5 Error: <code>cannot initialize a variable of type 'const int **' with an lvalue of type 'int **'<\/code>\uff0c\u610f\u5373 <code>int**<\/code> \u5230 <code>const int**<\/code> \u7684\u9690\u5f0f\u8f6c\u6362\u975e\u6cd5\u3002<\/p>\n\n\n\n<p>\u4f5c\u4e3a C++ \u7684\u5e38\u8bc6\uff0ctop-level qualifier \u5728\u8d4b\u503c\u65f6\u4f1a\u88ab\u5ffd\u7565\uff0c\u800c low-level qualifier \u53ea\u53ef\u589e\u4e0d\u53ef\u51cf\uff0c\u5982\u4e0b\u4f8b\uff1a<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"cpp\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">const int c{0};\nint n {c}; \/\/ const of c is ignored\n\nconst int* cp {nullptr};\nint* np {cp}; \/\/ error<\/pre>\n\n\n\n<p>\u7136\u800c\u4e8b\u5b9e\u4e0a\uff0c\u4e0a\u8ff0\u89c4\u5219\u4ec5\u4ec5\u9002\u7528\u4e8e\u4e00\u7ea7\u6307\u9488\u7684\u60c5\u51b5\u3002\u672c\u6587\u9996\u4f8b\u4e2d\uff0cppi2 \u7c7b\u578b\u7684\u6700\u5e95\u5c42\u88ab\u52a0\u4e0a const \u9650\u5b9a\uff0c\u4f46\u4f9d\u7136\u65e0\u6cd5\u63a5\u6536\u6765\u81ea <code>int**<\/code> \u7c7b\u578b\u5bf9\u8c61\u7684\u8d4b\u503c\u3002<\/p>\n\n\n\n<p>\u4e3a\u7406\u89e3\u539f\u56e0\uff0c\u4e0b\u9762\u628a\u9996\u4f8b\u62d3\u5c55\u6210\u4e3a\u5177\u6709\u7834\u574f\u6027\u7684\u4ee3\u7801\u3002<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"cpp\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">const int i {0};\nint *pi {nullptr};\nint **ppi {&amp;pi};\nconst int** ppi2 {const_cast&lt;const int**>(ppi)};\n*ppi2 = &i;\n**ppi = 1; \/\/ or *pi = 1;<\/pre>\n\n\n\n<p>\u7b2c\u56db\u884c\u4f7f\u7528 <code>const_cast<\/code> \u7ed5\u8fc7\u7f16\u8bd1\u5668\u7684\u68c0\u67e5\uff0c\u4e8e\u662f\u4fbf\u9020\u6210\u4e86\u60b2\u5267\uff1a\u672c\u6765\u4e0d\u80fd\u6307\u5411 <code>i<\/code> \u7684 <code>pi<\/code> \u4ee5\u53ca <code>ppi<\/code>\uff0c\u5728\u7ecf\u8fc7\u5e95\u5c42\u4e3a const \u4f46\u4e0a\u4e00\u7ea7\u7c7b\u578b\u65e0 const \u9650\u5b9a\u7684 <code>ppi2<\/code> \u53d8\u91cf\u7684\u7ed5\u8fc7\u540e\uff0c\u83b7\u5f97\u4e86 <code>i<\/code> \u7684\u5730\u5740\u3002\u56e0\u800c\u539f\u672c\u6709 <code>const<\/code> \u9650\u5b9a\u7684 <code>i<\/code> \u88ab\u6539\u53d8\u4e86\u503c\u3002<\/p>\n\n\n\n<p>\u901a\u8fc7\u8fd9\u4e2a\u4f8b\u5b50\u53ef\u4ee5\u611f\u53d7\u5230\uff0c\u5728\u6d89\u53ca\u591a\u7ea7\u6307\u9488\uff082 \u7ea7\u53ca\u4ee5\u4e0a\uff09\u7684\u60c5\u51b5\u4e0b\uff0c\u8d4b\u503c\u65f6\u7684\u9690\u5f0f\u7c7b\u578b\u8f6c\u6362\u4e2d\uff0c low-level qualifier \u5373\u4f7f\u589e\u52a0\u4e5f\u6709\u53ef\u80fd\u9020\u6210\u9690\u60a3\u3002<\/p>\n\n\n\n<p>\u56e0\u6b64\uff0c\u6307\u9488\u7c7b\u578b T1 \u5230 T2 \u7684\u7c7b\u578b\u8f6c\u6362\u80fd\u591f\u88ab\u5141\u8bb8\u7684\u60c5\u51b5\u9700\u6ee1\u8db3\uff1a<\/p>\n\n\n\n<ol class=\"wp-block-list\"><li>T1 \u4e0e T2 \u57fa\u7c7b\u578b\u4e00\u81f4\u4e14\u6307\u9488\u7ea7\u6570\u76f8\u540c\uff1b<\/li><li>\u8bb0\u76f8\u540c\u7684\u6307\u9488\u5c42\u7ea7\u4e3a n\uff0c\u540c\u65f6\u8bb0\u57fa\u7c7b\u578b\u6240\u5728\u7684\u7ea7\u522b\u4e3a n \u7ea7\uff0c\u6307\u9488\u53d8\u91cf\u672c\u8eab\u6240\u5728\u7684\u7ea7\u522b\u4e3a 0 \u7ea7\uff0c\u4e2d\u95f4\u5404\u7ea7\u6309\u987a\u5e8f\u6392\u5217\u3002\u90a3\u4e48\uff0c\u82e5 T1 \u7684 k \u7ea7\uff08k &gt; 0\uff09\u5b58\u5728\u67d0 qualifier\uff0cT2 \u7684 k \u7ea7\u4e5f\u9700\u8981\u6709\u76f8\u540c\u7684 quailifier\uff1b<\/li><li>\u5bf9\u4e8e\u6240\u6709\u7684 j &gt; 0\uff0c\u82e5 T2 \u7684 j \u7ea7\u5b58\u5728 T1 \u7684 j \u7ea7\u6240\u4e0d\u5b58\u5728\u7684 qualifier\uff0c\u90a3\u4e48 T2 \u4e2d\u6240\u6709\u7684 k \u7ea7\uff080 &lt; k &lt; j\uff09\u90fd\u8981\u6709 const \u9650\u5b9a\u3002<\/li><\/ol>\n\n\n\n<p>\u65e2\u7136\u7528\u8bcd\u662f qualifier\uff0c\u56e0\u6b64\u4e0d\u53ea\u5305\u62ec const\uff0c\u8fd8\u5305\u62ec volatile\u3002<\/p>\n\n\n\n<p>\u4e3a\u4e86\u4fbf\u4e8e\u7406\u89e3\uff0c\u4e3e\u4ee5\u4e0b\u51e0\u4e2a\u4f8b\u5b50\uff1a<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><code>T***<\/code> \u7c7b\u578b\u7684\u6307\u9488\u4e0d\u80fd\u8f6c\u6362\u4e3a <code>T*const*<\/code>\uff0c\u56e0\u4e3a\u4e0d\u6ee1\u8db3\u6761\u4ef6 1\uff1b<\/li><li><code>T*const**<\/code> \u4e0d\u80fd\u8f6c\u6362\u4e3a <code>T**const*<\/code>\uff0c\u56e0\u4e3a\u4e0d\u6ee1\u8db3\u6761\u4ef6 2\uff1b<\/li><li><code>T*const*****<\/code> \u4e0d\u80fd\u8f6c\u6362\u4e3a <code>T*const**volatile***<\/code> \u56e0\u4e3a\u4e0d\u6ee1\u8db3\u6761\u4ef6 3\uff1b\u4f46\u53ef\u4ee5\u8f6c\u6362\u4e3a <code>T*const**volatile*const*const volatile*<\/code> \u7c7b\u578b\u7684\u6307\u9488\u3002<\/li><\/ul>\n\n\n\n<p>\u4e8b\u5b9e\u4e0a\uff0c\u5bf9\u4e8e\u6b64\u7c7b\u884c\u4e3a\uff0cC++ 17 \u6807\u51c6 \u00a7 7.5 \u6709\u7740\u66f4\u8be6\u7ec6\u6e05\u695a\u7684\u89c4\u5b9a\u3002\u5e0c\u671b\u6709\u94b1\u540e\u80fd\u76f4\u63a5\u62dc\u8bfb\u539f\u6587\u3002\u7ed3\u5c3e\u5c06\u9644\u4e0a\u67d0\u7248\u8349\u7a3f\u94fe\u63a5\u4ee5\u4f9b\u53c2\u8003\u3002<\/p>\n\n\n\n<p>\u53c2\u8003\uff1a <a href=\"http:\/\/www.open-std.org\/jtc1\/sc22\/wg21\/docs\/papers\/2014\/n4296.pdf\">N4140 \u53f7\u8349\u7a3f<\/a> \uff08\u5176\u4e2d \u00a7 4.4 \u5bf9\u5e94 C++ 17 \u6807\u51c6 \u00a7 7.5\uff09<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u7f16\u8bd1\u5668\uff1a clang 6.0.0-1ubuntu2 \u81ea\u4f5c\u4e3b\u5f20\u7ffb\u8bd1\u4e86\u4e00\u4e0b\u3002\u4e09\u4e2a\u540d\u8bcd\u5bf9\u5e94\u7684\u539f\u6587\u5206\u522b\u662f multilevel pointer\u3001type qualifier \u4ee5\u53ca type conversion\u3002\u9009\u62e9\u7528\u4e09\u4e2a\u540d\u8bcd\u5e76\u5217\u4f5c\u4e3a\u6807\u9898\uff0c\u4e5f\u5b9e\u5728\u662f\u56e0\u4e3a\u6d89\u53ca\u7684\u5185\u5bb9\u6839\u672c\u65e0\u6cd5\u7528\u7b80\u77ed\u7684\u53e5\u5b50\u63cf\u8ff0\u51fa\u6765\u3002 \u4e0d\u59a8\u7528\u4e00\u4e2a\u4f8b\u5b50\u5f00\u5934\uff1a \u7f16\u8bd1\u62a5 Error: cannot initialize a variable of type &#8216;const int **&#8217; with an lvalue of type &#8216;int **&#8217;\uff0c\u610f\u5373 int** \u5230 const int** \u7684\u9690\u5f0f\u8f6c\u6362\u975e\u6cd5\u3002 \u4f5c\u4e3a C++ \u7684\u5e38\u8bc6\uff0ctop-level qualifier \u5728\u8d4b\u503c\u65f6\u4f1a\u88ab\u5ffd\u7565\uff0c\u800c low-level qualifier \u53ea\u53ef\u589e\u4e0d\u53ef\u51cf\uff0c\u5982\u4e0b\u4f8b\uff1a \u7136\u800c\u4e8b\u5b9e\u4e0a\uff0c\u4e0a\u8ff0\u89c4\u5219\u4ec5\u4ec5\u9002\u7528\u4e8e\u4e00\u7ea7\u6307\u9488\u7684\u60c5\u51b5\u3002\u672c\u6587\u9996\u4f8b\u4e2d\uff0cppi2 \u7c7b\u578b\u7684\u6700\u5e95\u5c42\u88ab\u52a0\u4e0a const \u9650\u5b9a\uff0c\u4f46\u4f9d\u7136\u65e0\u6cd5\u63a5\u6536\u6765\u81ea int** \u7c7b\u578b\u5bf9\u8c61\u7684\u8d4b\u503c\u3002 \u4e3a\u7406\u89e3\u539f\u56e0\uff0c\u4e0b\u9762\u628a\u9996\u4f8b\u62d3\u5c55\u6210\u4e3a\u5177\u6709\u7834\u574f\u6027\u7684\u4ee3\u7801\u3002 \u7b2c\u56db\u884c\u4f7f\u7528 const_cast \u7ed5\u8fc7\u7f16\u8bd1\u5668\u7684\u68c0\u67e5\uff0c\u4e8e\u662f\u4fbf\u9020\u6210\u4e86\u60b2\u5267\uff1a\u672c\u6765\u4e0d\u80fd\u6307\u5411 i \u7684 pi [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":259,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false},"version":2}},"categories":[14,17],"tags":[15,16],"class_list":["post-285","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-cpp","category-recommendation","tag-c","tag-16"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"https:\/\/www.eolstudy.com\/wp-content\/uploads\/2019\/09\/sumita-arora-c-plus-plus-pdf-download.jpg","jetpack_sharing_enabled":true,"jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/posts\/285"}],"collection":[{"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/comments?post=285"}],"version-history":[{"count":5,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/posts\/285\/revisions"}],"predecessor-version":[{"id":290,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/posts\/285\/revisions\/290"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/media\/259"}],"wp:attachment":[{"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/media?parent=285"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/categories?post=285"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.eolstudy.com\/index.php\/wp-json\/wp\/v2\/tags?post=285"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}