{"id":10312,"date":"2025-01-20T11:55:06","date_gmt":"2025-01-20T11:55:06","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=10312"},"modified":"2025-01-20T11:55:06","modified_gmt":"2025-01-20T11:55:06","slug":"stdformat-extension","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/stdformat-extension\/","title":{"rendered":"std::format Extension"},"content":{"rendered":"\n

Displaying the address of an arbitrary pointer in C++ 20 fails but succeeds with C++26.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

C++20<\/h3>\n\n\n\n

Only void<\/code>, const void<\/code>, and std::nullptr_t<\/code> pointer types are valid. If you want to display the address of an arbitrary pointer, you must cast it to (const) void*.<\/code><\/p>\n\n\n\n

\/\/ formatPointer20.cpp<\/span>\n\n#include <format><\/span>\n#include <iostream><\/span>\n#include <string><\/span>\n\nint<\/span> main<\/span>() {\n\n  double<\/span> d =<\/span> 123.456789<\/span>;\n\n  \/\/std::cout << "&d" << std::format("{}", &d) << '\\n';  <\/span>\n  std::<\/span>cout <<<\/span> "static_cast<void *>(&d): "<\/span> <<<\/span> std::<\/span>format("{}"<\/span>, static_cast<\/span><<\/span>void<\/span> *><\/span>(&<\/span>d)) <<<\/span> '\\n'<\/span>;     \n  std::<\/span>cout <<<\/span> "static_cast<const void *>(&d): "<\/span> <<<\/span> std::<\/span>format("{}"<\/span>, static_cast<\/span><<\/span>const<\/span> void<\/span> *><\/span>(&<\/span>d)) <<<\/span> '\\n'<\/span>; \n  std::<\/span>cout <<<\/span> "nullptr: "<\/span> <<<\/span> std::<\/span>format("{}"<\/span>, nullptr) <<<\/span> '\\n'<\/span>;           \n             \n}\n<\/pre><\/div>\n\n\n\n
In the main<\/code> function, a double<\/code> variable named d<\/code> is initialized with the value 123.456789<\/code>. The program then prints the address of this variable using std::format<\/code>.<\/p>\n\n\n\n
The first commented-out line attempts to print the address of d<\/code> directly using std::format(\"{}\", &d)<\/code>. This line is commented out because it would result in a compilation error. The std::format<\/code> function requires the pointer to be cast to void*<\/code> or const void*<\/code> to format it correctly.<\/p>\n\n\n\n
The next three lines demonstrate the correct way to format and print pointers using std::format<\/code>. The first of these lines prints the address of d<\/code> after casting it to void*<\/code>. The second line prints the address of d<\/code> after casting it to const void*<\/code>. The third line prints the value nullptr<\/code>, which represents a null pointer.<\/p>\n\n\n\n
Each of these lines uses std::cout<\/code> to print a descriptive message followed by the formatted pointer. The std::format<\/code> function is used to convert the pointer to a string representation that can be printed to the console.<\/p>\n\n\n\n
Here is the output of the program: <\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Enabling the commented-out line causes a long error message:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
C++26 <\/h3>\n\n\n\n
With C++26, you can directly output the pointer:<\/p>\n\n\n\n
\/\/ formatPointer26.cpp<\/span>\n\n#include <format><\/span>\n#include <iostream><\/span>\n#include <string><\/span>\n\nint<\/span> main<\/span>() {\n\n  double<\/span> d =<\/span> 123.456789<\/span>;\n  double<\/span>*<\/span> p =<\/span> &<\/span>d;\n\n  std::<\/span>cout <<<\/span> "&d"<\/span> <<<\/span> std::<\/span>format("{:P}"<\/span>, p) <<<\/span> '\\n'<\/span>;  \n \n}\n<\/pre><\/div>\n\n\n\n
Honesty, the program does not compile. This contradicts compiler support for C++26<\/a> and the proposal P2510R3<\/a>. Do you know why?<\/p>\n\n\n\n
A solution will be provided in the next article.<\/p>\n\n\n\n
What’s Next?<\/h2>\n\n\n\n
Concurrency will get with C++26 two nice features for lock-free data structures: Read-copy update and hazard pointers. <\/p>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
Displaying the address of an arbitrary pointer in C++ 20 fails but succeeds with C++26. C++20 Only void, const void, and std::nullptr_t pointer types are valid. If you want to display the address of an arbitrary pointer, you must cast it to (const) void*. \/\/ formatPointer20.cpp #include <format> #include <iostream> #include <string> int main() { […]<\/p>\n","protected":false},"author":21,"featured_media":10313,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[559],"tags":[454],"class_list":["post-10312","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c26-blog","tag-format"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10312","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=10312"}],"version-history":[{"count":13,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10312\/revisions"}],"predecessor-version":[{"id":10568,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10312\/revisions\/10568"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/10313"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=10312"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=10312"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=10312"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}