In this post, we will discuss rvalue references and move semantics in C++11.<\/p>\n
Rvalue references and move semantics can be very complex, see this article<\/a>. But the truth is that: the simplest is the best<\/strong>. It starts with three examples of returning values from functions. Read them and ask yourself which of them is going to do no vector copy.<\/p>\n It returns a temporary caught by Actually it will cause a run time error. Because This example is roughly equivalent to the first. Actually the The following simple code is the best way to get value from functions.<\/p>\n It will actually do Return value optimization, aka RVO, is a compiler optimization technique that allows the compiler to construct the return value of a function at the call site. The technique is also named “elision”. C++98\/03 standard doesn\u2019t require the compiler to provide RVO optimization, but most popular C++ compilers contain this optimization technique, such as Summary In this post, we will discuss rvalue references and move semantics in C++11. Details Rvalue references and move semantics can be very complex, see this article. But the truth is that: the simplest is the best. It starts with three examples of returning values from functions. Read them and ask yourself which of them…<\/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-187","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\/187","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=187"}],"version-history":[{"count":3,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/187\/revisions"}],"predecessor-version":[{"id":190,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/posts\/187\/revisions\/190"}],"wp:attachment":[{"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/media?parent=187"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/categories?post=187"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/nanzhou.cc\/index.php\/wp-json\/wp\/v2\/tags?post=187"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}First example<\/h4>\n
std::vector<int> return_vector(void) {\n std::vector<int> tmp {1,2,3,4,5};\n return tmp;\n}\n\nstd::vector<int> &&rval_ref = return_vector();\n<\/code><\/pre>\nrval_ref<\/code> whose life extended beyond the rval_ref<\/code> definition. We can use it as if you had caught it by value.<\/p>\nSecond example<\/h4>\n
std::vector<int>&& return_vector(void) {\n std::vector<int> tmp {1,2,3,4,5};\n return std::move(tmp);\n}\n\nstd::vector<int> &&rval_ref = return_vector();\n<\/code><\/pre>\nrval_ref<\/code> now holds a reference to the destructed tmp inside the function. The following codes crashes.<\/p>\nstd::vector<int> &&rval_ref = return_vector();\ncout << rval_ref[0] << endl;\nrval_ref[0] = 2;\ncout << rval_ref[0] << endl; \/\/ Process finished with exit code 11\n<\/code><\/pre>\nThird example<\/h4>\n
std::vector<int> return_vector(void) {\n std::vector<int> tmp {1,2,3,4,5};\n return std::move(tmp);\n}\nstd::vector<int> &&rval_ref = return_vector();\n<\/code><\/pre>\nstd::move<\/code> on tmp<\/code> is unnecessary and even can be inhibit return value optimization.<\/p>\nBest way<\/h4>\n
std::vector<int> return_vector(void)\n{\n std::vector<int> tmp {1,2,3,4,5};\n return tmp;\n}\n\nstd::vector<int> rval_ref = return_vector();\n<\/code><\/pre>\n
\n1. RVO (no copy, no move) by default;
\n2. move constructor if RVO is not possible;
\n3. copy constructor if RVO is not possible and the returned type did not have a move constructor.
\nThanks to the compilers, “often the cleanest, simplest code that doesn\u2019t even mention move<\/code> or &&<\/code> anywhere is just what you want \u2013 that\u2019s C++11, clean, safe, and faster than ever”.<\/p>\nRVO<\/h4>\n
IBM XL C++ compiler<\/code>, GCC<\/code> and Clang<\/code> . This optimization technique is included in the C++11 standard due to its prevalence. As defined in Section 12.8 in the C++11 standard, the name of the technique is “copy elision”.<\/p>\nReference<\/h2>\n
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