{"id":2295,"date":"2025-02-15T22:21:32","date_gmt":"2025-02-15T22:21:32","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=2295"},"modified":"2025-04-04T19:23:39","modified_gmt":"2025-04-04T19:23:39","slug":"lambdas","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/lambdas\/","title":{"rendered":"Lambdas & Closures"},"content":{"rendered":"\n

<\/span>What’s the Problem?<\/span><\/h2>\n\n\n\n

Remember the example we used, when we were talking about Function Objects in the Standard Template Library topic?<\/p>\n\n\n\n

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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  STL Function Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/23\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span><vector><\/span>  <\/span>\/\/ For vector<\/span><\/span>\n#include<\/span> <\/span><numeric><\/span>  <\/span>\/\/ For accumulate()<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>Multiply<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>int<\/span> <\/span>operator()<\/span>(<\/span>int<\/span> <\/span>x<\/span>, <\/span>int<\/span> <\/span>y<\/span>) <\/span>const<\/span> { <\/span>return<\/span> x * y; }<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[])<\/span><\/span>\n{<\/span><\/span>\n  <\/span>int<\/span> values[] = {<\/span>2<\/span>, <\/span>3<\/span>, <\/span>5<\/span>, <\/span>7<\/span>, <\/span>11<\/span>};<\/span><\/span>\n  <\/span>size_t<\/span> len = <\/span>sizeof<\/span>(values)\/<\/span>sizeof<\/span>(<\/span>values<\/span>[<\/span>0<\/span>]);<\/span><\/span>\n  <\/span><\/span>\n  \/\/ Initialize vector using pointer iterator<\/span><\/span>\n  <\/span>std<\/span>::vector<<\/span>int<\/span>> <\/span>v<\/span>(&<\/span>values<\/span>[<\/span>0<\/span>], &<\/span>values<\/span>[len]);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>int<\/span> product = <\/span>accumulate<\/span>(<\/span>v<\/span>.<\/span>begin<\/span>(), <\/span>v<\/span>.<\/span>end<\/span>(), <\/span>1<\/span>, <\/span>Multiply<\/span>());<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Result: "<\/span> << product << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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which outputs the following:<\/p>\n\n\n\n
Result: 2310\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n\n\n\n
where we provided a class that acted as a function object because it defined an operator()<\/strong> method.<\/p>\n<\/div>\n<\/div>\n\n\n\n
<\/span>Enter Lambdas<\/span><\/h2>\n\n\n\n
Sometimes it’s annoying to have to create an entire class to perform something that is only used once, and locally.<\/p>\n\n\n\n
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Beware!<\/strong><\/p>\n\n\n\n
You are entering a C++ realm of strange, weird, arcane, and abbreviated syntax.<\/p>\n<\/blockquote>\n\n\n\n
So, we can do the following:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Lambdas<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 2\/15\/2025.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span><vector><\/span>  <\/span>\/\/ For vector<\/span><\/span>\n#include<\/span> <\/span><numeric><\/span>  <\/span>\/\/ For accumulate()<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[])<\/span><\/span>\n{<\/span><\/span>\n  <\/span>int<\/span> values[] = {<\/span>2<\/span>, <\/span>3<\/span>, <\/span>5<\/span>, <\/span>7<\/span>, <\/span>11<\/span>};<\/span><\/span>\n  <\/span>size_t<\/span> len = <\/span>sizeof<\/span>(values)\/<\/span>sizeof<\/span>(<\/span>values<\/span>[<\/span>0<\/span>]);<\/span><\/span>\n  <\/span><\/span>\n  \/\/ Initialize vector using pointer iterator<\/span><\/span>\n  <\/span>std<\/span>::vector<<\/span>int<\/span>> <\/span>v<\/span>(&<\/span>values<\/span>[<\/span>0<\/span>], &<\/span>values<\/span>[len]);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>int<\/span> product = <\/span>accumulate<\/span>(<\/span>v<\/span>.<\/span>begin<\/span>(), <\/span>v<\/span>.<\/span>end<\/span>(), <\/span>1<\/span>, <\/span><\/span>\n                           [](<\/span>int<\/span> <\/span>x<\/span>, <\/span>int<\/span> <\/span>y<\/span>) \/\/ Our lambda<\/span><\/span>\n                           {<\/span><\/span>\n                             <\/span>return<\/span> x * y;<\/span><\/span>\n                           }<\/span><\/span>\n                          );<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Result: "<\/span> << product << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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which also outputs the same result:<\/p>\n\n\n\n
Result: 2310\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n\n\n\n
So, what did we do?<\/p>\n<\/div>\n<\/div>\n\n\n\n
What is that really strange syntax? It’s a lambda expression<\/em><\/strong>.<\/p>\n\n\n\n
<\/span>Lambda Expressions<\/span><\/h3>\n\n\n\n
Aside from the eleventh letter of the Greek alphabet ( \"\"), what is a Lambda<\/em><\/strong>?.<\/p>\n\n\n\n
A lambda expression<\/strong> (also called a lambda<\/strong> or closure<\/strong>) allows us to define an anonymous function inside another function. <\/p>\n\n\n\n
The nesting is important, as it allows us both to avoid namespace naming pollution, and to define the function as close to where it is used as possible.<\/p>\n\n\n\n
The syntax of a Lambda Expression is:<\/p>\n\n\n\n
[ capture-clause<\/em> ] ( parameter-lis<\/em>t ) -> return-type<\/em> { statements<\/em>; }<\/strong><\/code><\/pre>\n\n\n\n