{"id":3967,"date":"2019-01-07T19:30:02","date_gmt":"2019-01-07T18:30:02","guid":{"rendered":"http:\/\/roboblog.fatal-fury.de\/?p=3967"},"modified":"2019-01-07T19:30:02","modified_gmt":"2019-01-07T18:30:02","slug":"c-guns-pass-function-objects-per-forwarding-references","status":"publish","type":"post","link":"http:\/\/roboblog.fatal-fury.de\/?p=3967","title":{"rendered":"C++ Guns: Pass function objects per Forwarding references!"},"content":{"rendered":"

I say: Pass (template) function objects per Forwarding references!<\/strong> and not per value. The reason is simple: it works also with function objects with state.<\/p>\n

https:\/\/en.cppreference.com\/w\/cpp\/language\/reference<\/a><\/p>\n

A little example will show it:<\/p>\n

\r\n#include <iostream>\r\n#include <random>\r\nusing namespace std;\r\n\r\ntemplate<typename Function>\r\nauto perValue(Function f) {\r\n  return f();\r\n}\r\n\r\ntemplate<typename Function>\r\nauto perForwardReference(Function&& f) {\r\n  return f();\r\n}\r\n\r\nint main() {\r\n  std::minstd_rand gen(1156823295);\r\n  \r\n  cout << "Generate 5 random numbers:\\n";\r\n  cout << "calls to perValue:\\n";\r\n  for(int i=0; i < 5; i++) {\r\n    cout << perValue(gen) << "\\n";\r\n  }\r\n  \r\n  cout << "calls to perForwardReference:\\n";\r\n  for(int i=0; i < 5; i++) {\r\n    cout << perForwardReference(gen) << "\\n";\r\n  }  \r\n}\r\n<\/pre>\n
\nGenerate 5 random numbers:
\ncalls to perValue:
\n4
\n4
\n4
\n4
\n4
\ncalls to perForwardReference:
\n4
\n193084
\n730423176
\n870612250
\n1216431607\n<\/p><\/blockquote>\n
\"\"<\/a><\/p>\n
It doesn\u2019t work because the random number generate object has a state. The call to function perValue()<\/em> change this state on a copy of the generator. And not the one, we pass as argument. But there is no compile time error. Not a single warning. That's the worst case IMO.<\/p>\n
The C++ standard and Scott Meyers book \"Effective STL\" recommend to pass by value. IMO for historical reasons.<\/p>\n
\nC++11 \u00a725.1\/10:
\n    [ Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object such as reference_wrapper (20.8.3), or some equivalent solution. \u2014end note ]\n<\/p><\/blockquote>\n
stackoverflow: why function objects should be pass-by-value<\/a><\/p>\n
The standard should be updated and the book is horrible outdated. It was written 2008 and forward references come with C++2011. The old solution is to wrap the function into something which can be pass by value, but also change the state. We have now (2011) std::function<\/em> for that task. And not this ugly thing in the book.<\/p>\n","protected":false},"excerpt":{"rendered":"
I say: Pass (template) function objects per Forwarding references! and not per value. The reason is simple: it works also with function objects with state. https:\/\/en.cppreference.com\/w\/cpp\/language\/reference A little example will show it: #include <iostream> #include <random> using namespace std; template<typename Function> auto perValue(Function f) { return f(); } template<typename Function> auto perForwardReference(Function&& f) { return […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[17],"class_list":["post-3967","post","type-post","status-publish","format-standard","hentry","category-allgemein","tag-cpp"],"_links":{"self":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3967","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3967"}],"version-history":[{"count":11,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3967\/revisions"}],"predecessor-version":[{"id":3978,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3967\/revisions\/3978"}],"wp:attachment":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3967"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3967"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3967"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}