{"id":108187,"date":"2023-05-15T07:00:00","date_gmt":"2023-05-15T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108187"},"modified":"2023-05-16T10:57:29","modified_gmt":"2023-05-16T17:57:29","slug":"20230515-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230515-00\/?p=108187","title":{"rendered":"What are the duck-typing requirements of C++\/WinRT com_ptr<\/CODE>?"},"content":{"rendered":"

We conclude our survey of duck-typing requirements of various C++ COM smart pointer libraries by looking at C++\/WinRT’s com_ptr<\/code>, running it through our standard tests.<\/p>\n

\/\/ Dummy implementations of AddRef and Release for\r\n\/\/ testing purposes only. In real code, they would\r\n\/\/ manage the object reference count.\r\nstruct Test\r\n{\r\n    void AddRef() {}\r\n    void Release() {}\r\n    Test* AddressOf() { return this; }\r\n};\r\n\r\nstruct Other\r\n{\r\n    void AddRef() {}\r\n    void Release() {}\r\n};\r\n\r\n\/\/ Pull in the smart pointer library\r\n\/\/ (this changes based on library)\r\n#include <winrt\/base.h><\/span>\r\n\u00a0\r\nusing TestPtr = winrt::com_ptr<Test>;\u00a0\u00a0<\/span>\r\nusing OtherPtr = winrt::com_ptr<Other>;<\/span>\r\n\r\nvoid test()\r\n{\r\n    Test test;\r\n\r\n    \/\/ Default construction\r\n    TestPtr ptr;\r\n\r\n    \/\/ Construction from raw pointer\r\n    TestPtr ptr2(&test); \/\/ (does not compile)\r\n\r\n    \/\/ Copy construction\r\n    TestPtr ptr3(ptr2);\r\n\r\n    \/\/ Attaching and detaching\r\n    auto p = ptr3.detach<\/span>();\r\n    ptr.attach<\/span>(p);\r\n\r\n    \/\/ Assignment from same-type raw pointer\r\n    ptr3.copy_from(&test);<\/span>\r\n\r\n    \/\/ Assignment from same-type smart pointer\r\n    ptr3 = ptr;\r\n\r\n    \/\/ Accessing the wrapped object\r\n    \/\/ (this changes based on library)\r\n    if (ptr.get()<\/span> != &test) {\r\n        std::terminate(); \/\/ oops\r\n    }\r\n    if (ptr->AddressOf() != &test) {\r\n        std::terminate(); \/\/ oops\r\n    }\r\n\r\n    \/\/ Returning to empty state\r\n    ptr3 = nullptr;\r\n\r\n    \/\/ Receiving a new pointer\r\n    \/\/ (this changes based on library)\r\n    Test** out = ptr3.put()<\/span>;\r\n\r\n    \/\/ Bonus: Comparison.\r\n    if (ptr == ptr2) {}\r\n    if (ptr != ptr2) {}\r\n    if (ptr < ptr2) {}\r\n\r\n    \/\/ Litmus test: Accidentally bypassing the wrapper\r\n    ptr->AddRef();\r\n    ptr->Release();\r\n\r\n    \/\/ Litmus test: Construction from other-type raw pointer\r\n    Other other;\r\n    TestPtr ptr4(&other);\r\n\r\n    \/\/ Litmus test: Construction from other-type smart pointer\r\n    OtherPtr optr;\r\n    TestPtr ptr5(optr);\r\n\r\n    \/\/ Litmus test: Assignment from other-type raw pointer\r\n    ptr.copy_from(&other);<\/span>\r\n\r\n    \/\/ Litmus test: Assignment from other-type smart pointer\r\n    ptr = optr;\r\n\r\n    \/\/ Destruction\r\n}\r\n<\/pre>\n
C++\/WinRT doesn’t require that the Release<\/code> method return a reference count, unlike ATL, WRL, and wil. So that’s a relief.<\/p>\n
As with wil, we have to make a small tweak to the boilerplate by switching to lowercase names for detach<\/code> and attach<\/code>, because that’s how C++\/WinRT spells them.<\/p>\n
Another thing we have to fix is removing construction from raw pointers. C++\/WinRT doesn’t support the operation of “construct with shared ownership of a raw pointer”. It does support “take ownership of a raw pointer” by passing the marker winrt::take_ownership_of_abi<\/code> as a second parameter. However, this is not generally used because it also discards type safety.<\/p>\n
Instead of assigning a raw pointer, C++\/WinRT uses the copy_from<\/code> method. This makes it clearer that the smart pointer is sharing ownership with the original, rather than taking ownership from it. (The attach<\/code> method takes ownership.)<\/p>\n
The only way to receive a pointer in C++\/WinRT is to use the put<\/code> method. This releases the old pointer and nulls it out, then returns the address of the pointer so a new value can be placed there. There is no ability to access the inner pointer for in\/out use.<\/p>\n
C++\/WinRT doesn’t “color” the return value of the -><\/code> operator, so you don’t get protection from signatures, but you also don’t get protection from accidentally doing a ptr->Release()<\/code> when you meant to do a ptr = nullptr<\/code>, but the two expressions are so different-looking that you’re less likely to confuse them.<\/p>\n
The other-type litmus tests all pass. They all result in various types of compile-time errors.<\/p>\n
Finally, so here’s the scorecard for winrt::com_ptr<\/code>.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
winrt::com_ptr<\/code> scorecard<\/th>\n<\/tr>\n
Default construction<\/td>\nPass<\/td>\n<\/tr>\n
Construct from raw pointer<\/td>\nNot supported<\/td>\n<\/tr>\n
Copy construction<\/td>\nPass<\/td>\n<\/tr>\n
Destruction<\/td>\nPass<\/td>\n<\/tr>\n
Attach and detach<\/td>\nPass<\/td>\n<\/tr>\n
Assign to same-type raw pointer<\/td>\nPass (copy_from<\/code>)<\/td>\n<\/tr>\n
Assign to same-type smart pointer<\/td>\nPass<\/td>\n<\/tr>\n
Fetch the wrapped pointer<\/td>\nget()<\/code><\/td>\n<\/tr>\n
Access the wrapped object<\/td>\n-><\/code><\/td>\n<\/tr>\n
Receive pointer via &<\/code><\/td>\nN\/A<\/td>\n<\/tr>\n
Release and receive pointer<\/td>\nput()<\/code><\/td>\n<\/tr>\n
Preserve and receive pointer<\/td>\nN\/A<\/td>\n<\/tr>\n
Return to empty state<\/td>\nPass<\/td>\n<\/tr>\n
Comparison<\/td>\nPass<\/td>\n<\/tr>\n
Accidental bypass<\/td>\nFail<\/td>\n<\/tr>\n
Construct from other-type raw pointer<\/td>\nPass<\/td>\n<\/tr>\n
Construct from other-type smart pointer<\/td>\nPass<\/td>\n<\/tr>\n
Assign from other-type raw pointer<\/td>\nPass<\/td>\n<\/tr>\n
Assign from other-type smart pointer<\/td>\nPass<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Next time, we’ll capture all these results into a large comparison table and discuss what we find.<\/p>\n","protected":false},"excerpt":{"rendered":"
The experiments conclude.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-108187","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
The experiments conclude.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108187","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=108187"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108187\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=108187"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108187"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108187"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}