{"id":5204,"date":"2017-06-28T20:22:46","date_gmt":"2017-06-28T20:22:46","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=5204"},"modified":"2017-06-28T20:23:38","modified_gmt":"2017-06-28T20:23:38","slug":"c-lambdas-in-a-little-more-depth","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/c-lambdas-in-a-little-more-depth\/","title":{"rendered":"C++ lambda’s in a little more depth"},"content":{"rendered":"

Lambda expressions<\/a> appeared in C++ 11.<\/p>\n

Let’s take a simple example. We want to create a lambda which takes an enum (of type UpdateFlag) and returns a LPTSTR (yes we’re in Visual C++ world) which we’ll use when we writing to cout.<\/p>\n

We can create the lambda like this<\/p>\n

\r\nauto lambda = [](UpdateFlag flag)\r\n{\r\n   switch (flag)\r\n   {\r\n      case UpdateFlag::Update:\r\n         return _T("Update");\r\n      case UpdateFlag::Delete:\r\n         return _T("Delete");\r\n      case UpdateFlag::Missing:\r\n         return _T("Missing");\r\n   }\r\n};\r\n<\/pre>\n
if you prefer to not use the auto<\/em> keyword we can use the functional<\/em> code like this<\/p>\n
\r\nstd::function<LPCTSTR(UpdateFlag)> lambda = [](UpdateFlag flag)\r\n{\r\n\/\/ switch removed for brevity\r\n};\r\n<\/pre>\n
I’m not quite sure where you’d need to use the std::function variation of this, but this is what it would look like.<\/em><\/p>\n
Now to use our lambda, we’d simply have something like<\/p>\n
\r\nstd::cout << lamba(updateFlag);\r\n<\/pre>\n
We can also pass the lambda (without creating a variable to it) using the following<\/p>\n
\r\nstd::cout << [](UpdateFlag flag)\r\n{\r\n\/\/ switch removed for brevity\r\n}(updateFlag);\r\n<\/pre>\n
Note: we need to pass the updateFlag in a parameter list after the lamba declaration.<\/em><\/p>\n
The [] is an empty capture list, we could alter the previous samples (assuming a variable updateFlag is available within the scope of the lambda) by passing the updateFlag as part of the capture list, for example<\/p>\n
\r\nauto updateFlag = UpdateFlag::Update;\r\n\r\nauto lambda = [updateFlag]\r\n{\r\n   switch (updateFlag)\r\n   {\r\n      case UpdateFlag::Update:\r\n         return _T("Update");\r\n      case UpdateFlag::Delete:\r\n         return _T("Delete");\r\n      case UpdateFlag::Missing:\r\n         return _T("Missing");\r\n   }\r\n};\r\n<\/pre>\n
Structuring the lamba<\/strong><\/p>\n
Let’s look at the format of the lamba…<\/p>\n
The lamba syntax basically takes the form<\/p>\n