{"id":5951,"date":"2020-07-31T08:14:25","date_gmt":"2020-07-31T08:14:25","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/more-powerful-lambdas-with-c-20\/"},"modified":"2023-06-26T09:46:57","modified_gmt":"2023-06-26T09:46:57","slug":"more-powerful-lambdas-with-c-20","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/more-powerful-lambdas-with-c-20\/","title":{"rendered":"More Powerful Lambdas with C++20"},"content":{"rendered":"

Thanks to C++20, lambdas have become more powerful. From the various lambda improvements, template parameters for lambdas are my favorite ones.<\/p>\n

<\/p>\n

 \"TimelineCpp20CoreLanguage\"Lambdas support with C++20 template parameters can be default-constructed and support copy-assignment when they have no state and can be used in unevaluated contexts. Additionally, they detect when you implicitly copy the this<\/span> pointer. This means a significant cause of undefined behavior with lambdas is gone.<\/p>\n

Let’s start with template parameters for lambdas.<\/p>\n<\/p>\n

Template Parameter for Lambdas<\/h2>\n

Admittedly, the differences between typed lambdas, generic lambdas, and template lambdas (template parameters for lambdas) are subtle.<\/p>\n

Four lambda variations<\/h3>\n

The following program presents four variations of the add<\/span> function using lambdas for their implementation.<\/p>\n

 <\/p>\n

\n
\/\/ templateLambda.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n#include <string><\/span>\r\n#include <vector><\/span>\r\n\r\nauto<\/span> sumInt =<\/span> [](int<\/span> fir, int<\/span> sec) { return<\/span> fir +<\/span> sec; };            \/\/ only to int convertible types    (C++11)<\/span>\r\nauto<\/span> sumGen =<\/span> [](auto<\/span> fir, auto<\/span> sec) { return<\/span> fir +<\/span> sec; };          \/\/ arbitrary types                  (C++14)<\/span>\r\nauto<\/span> sumDec =<\/span> [](auto<\/span> fir, decltype(fir) sec) { return<\/span> fir +<\/span> sec; }; \/\/ arbitrary, but convertible types (C++14)<\/span>\r\nauto<\/span> sumTem =<\/span> []<<\/span>typename<\/span> T><\/span>(T fir, T sec) { return<\/span> fir +<\/span> sec; };    \/\/ arbitrary, but identical types   (C++20)<\/span>\r\n\r\nint<\/span> main<\/span>() {\r\n    \r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n                                                                            \/\/ (1)<\/span>\r\n    std::<\/span>cout <<<\/span> \"sumInt(2000, 11): \"<\/span> <<<\/span> sumInt(2000<\/span>, 11<\/span>) <<<\/span> std::<\/span>endl;  \r\n    std::<\/span>cout <<<\/span> \"sumGen(2000, 11): \"<\/span> <<<\/span> sumGen(2000<\/span>, 11<\/span>) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumDec(2000, 11): \"<\/span> <<<\/span> sumDec(2000<\/span>, 11<\/span>) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumTem(2000, 11): \"<\/span> <<<\/span> sumTem(2000<\/span>, 11<\/span>) <<<\/span> std::<\/span>endl;\r\n    \r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n                                                                            \/\/ (2)<\/span>\r\n    std::<\/span>string hello =<\/span> \"Hello \"<\/span>;\r\n    std::<\/span>string world =<\/span> \"world\"<\/span>; \r\n    \/\/ std::cout << \"sumInt(hello, world): \" << sumInt(hello, world) << std::endl; ERROR<\/span>\r\n    std::<\/span>cout <<<\/span> \"sumGen(hello, world): \"<\/span> <<<\/span> sumGen(hello, world) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumDec(hello, world): \"<\/span> <<<\/span> sumDec(hello, world) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumTem(hello, world): \"<\/span> <<<\/span> sumTem(hello, world) <<<\/span> std::<\/span>endl;\r\n    \r\n    \r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n                                                                             \/\/ (3)<\/span>\r\n    std::<\/span>cout <<<\/span> \"sumInt(true, 2010): \"<\/span> <<<\/span> sumInt(true<\/span>, 2010<\/span>) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumGen(true, 2010): \"<\/span> <<<\/span> sumGen(true<\/span>, 2010<\/span>) <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"sumDec(true, 2010): \"<\/span> <<<\/span> sumDec(true<\/span>, 2010<\/span>) <<<\/span> std::<\/span>endl; <\/span> \r\n    \/\/ std::cout << \"sumTem(true, 2010): \" << sumTem(true, 2010) << std::endl; ERROR<\/span>\r\n    \r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n    \r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Before I show the presumably astonishing output of the program, I want to compare the four lambdas.<\/p>\n