{"id":2183,"date":"2025-01-28T20:02:49","date_gmt":"2025-01-28T20:02:49","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=2183"},"modified":"2025-01-28T21:39:07","modified_gmt":"2025-01-28T21:39:07","slug":"function-objects","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/the-standard-template-library-2\/function-objects\/","title":{"rendered":"Function Objects"},"content":{"rendered":"\n

Function Objects<\/h2>\n\n\n\n

In the previous example, we are passing the address of a function (mult()<\/strong>) into the accumulate()<\/strong> function.<\/p>\n\n\n\n

We can instead use a Function Object<\/strong><\/em>:<\/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 produces the following:<\/p>\n\n\n\n
Result: 2310\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
A function object<\/em>, or functor<\/em>, is any type that implements operator()<\/strong>. This operator is referred to as the call operator<\/em> or sometimes the application operator<\/em>. The C++ Standard Library uses function objects primarily as sorting criteria for containers and in algorithms.<\/p>\n\n\n\n
Reasons for using a function object instead of an ordinary function include:<\/p>\n\n\n\n