I recently discovered a C++ technique I haven’t seen recognized before in any books, articles, or mentioned online anywhere (search terms are difficult perhaps). For want of a better name I call it Structural Capture. Consider the following code:<\/p>\n
#include \r\n#include \r\nusing namespace std;\r\n\r\n\/\/---------------------------------------------------------------\r\nstruct Foo\r\n{\r\n template \r\n Foo(const T& t)\r\n {\r\n m_outputFunc = [=] (ostream& s) -> ostream& { return s << t; };\r\n };\r\n\r\n function m_outputFunc;\r\n};\r\n\r\nostream& operator<<(ostream& s, const Foo& f)\r\n{\r\n return f.m_outputFunc(s);\r\n}\r\n\r\n\/\/---------------------------------------------------------------\r\nint main(int argc, char *argv[])\r\n{\r\n Foo f(1);\r\n cout << f;\r\n\r\n return 0;\r\n}<\/pre>\nFoo is a regular struct(\/class), but its constructor is a method template which wraps its argument in a lambda. When you construct a Foo, the you get the benefit of structural typing that templates bring: as long as the template argument implements a stream output operator, it'll compile.<\/p>\n
But through the use of a lambda, you also get the benefit of runtime polymorphism, and because Foo is a concrete type, you can put it in a data structure (like a container) without the template argument leaking out.<\/p>\n
This seems like it could be quite useful in various scenarios. Sure, there's a memory cost, and you could look at it as replacing a (per-type) vtable with per-instance member variables (ie. lambda functions and their associated captures). But as a way of mixing the benefits of compile-time and runtime polymorphism I think it's pretty neat.<\/p>\n","protected":false},"excerpt":{"rendered":"
I recently discovered a C++ technique I haven’t seen recognized before in any books, articles, or mentioned online anywhere (search terms are difficult perhaps). For want of a better name I call it Structural Capture. Consider the following code: #include #include using namespace std; \/\/————————————————————— struct Foo { template Foo(const…<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[22,8],"tags":[],"class_list":["post-886","post","type-post","status-publish","format-standard","hentry","category-cpp","category-programming"],"_links":{"self":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/886","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=886"}],"version-history":[{"count":6,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/886\/revisions"}],"predecessor-version":[{"id":892,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/886\/revisions\/892"}],"wp:attachment":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=886"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=886"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=886"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}