{"id":3884,"date":"2022-08-20T21:58:23","date_gmt":"2022-08-20T16:28:23","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-why-can-templates-only-be-implemented-in-the-header-file\/"},"modified":"2022-08-20T21:58:23","modified_gmt":"2022-08-20T16:28:23","slug":"solved-why-can-templates-only-be-implemented-in-the-header-file","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-why-can-templates-only-be-implemented-in-the-header-file\/","title":{"rendered":"(Solved) Why can templates only be implemented in the header file?"},"content":{"rendered":"<p> [ad_1]<br \/>\n<\/p>\n<div id=\"answer-495056\" class=\"answer js-answer accepted-answer js-accepted-answer\" data-answerid=\"495056\" data-parentid=\"495021\" data-score=\"1859\" data-position-on-page=\"1\" data-highest-scored=\"1\" data-question-has-accepted-highest-score=\"1\" itemprop=\"acceptedAnswer\" itemscope itemtype=\"https:\/\/schema.org\/Answer\">\n<div class=\"post-layout\">\n<div class=\"votecell post-layout--left\"><\/div>\n<div class=\"answercell post-layout--right\">\n<div class=\"s-prose js-post-body\" itemprop=\"text\">\n<p>Caveat: It is <em>not<\/em> necessary to put the implementation in the header file, see the alternative solution at the end of this answer.<\/p>\n<p>Anyway, the reason your code is failing is that, when instantiating a template, the compiler creates a new class with the given template argument. For example:<\/p>\n<pre><code>template&lt;typename T&gt;\nstruct Foo\n{\n    T bar;\n    void doSomething(T param) {\/* do stuff using T *\/}\n};\n\n\/\/ somewhere in a .cpp\nFoo&lt;int&gt; f; \n<\/code><\/pre>\n<p>When reading this line, the compiler will create a new class (let&#8217;s call it <code>FooInt<\/code>), which is equivalent to the following:<\/p>\n<pre><code>struct FooInt\n{\n    int bar;\n    void doSomething(int param) {\/* do stuff using int *\/}\n}\n<\/code><\/pre>\n<p>Consequently, the compiler needs to have access to the implementation of the methods, to instantiate them with the template argument (in this case <code>int<\/code>). If these implementations were not in the header, they wouldn&#8217;t be accessible, and therefore the compiler wouldn&#8217;t be able to instantiate the template.<\/p>\n<p>A common solution to this is to write the template declaration in a header file, then implement the class in an implementation file (for example .tpp), and include this implementation file at the end of the header.<\/p>\n<p>Foo.h<\/p>\n<pre><code>template &lt;typename T&gt;\nstruct Foo\n{\n    void doSomething(T param);\n};\n\n#include \"Foo.tpp\"\n<\/code><\/pre>\n<p>Foo.tpp<\/p>\n<pre><code>template &lt;typename T&gt;\nvoid Foo&lt;T&gt;::doSomething(T param)\n{\n    \/\/implementation\n}\n<\/code><\/pre>\n<p>This way, implementation is still separated from declaration, but is accessible to the compiler.<\/p>\n<h1>Alternative solution<\/h1>\n<p>Another solution is to keep the implementation separated, and explicitly instantiate all the template instances you&#8217;ll need:<\/p>\n<p>Foo.h<\/p>\n<pre><code>\/\/ no implementation\ntemplate &lt;typename T&gt; struct Foo { ... };\n<\/code><\/pre>\n<p>Foo.cpp<\/p>\n<pre><code>\/\/ implementation of Foo's methods\n\n\/\/ explicit instantiations\ntemplate class Foo&lt;int&gt;;\ntemplate class Foo&lt;float&gt;;\n\/\/ You will only be able to use Foo with int or float\n<\/code><\/pre>\n<p>If my explanation isn&#8217;t clear enough, you can have a look at the <a rel=\"nofollow noopener\" target=\"_blank\" href=\"https:\/\/isocpp.org\/wiki\/faq\/templates#templates-defn-vs-decl\">C++ Super-FAQ on this subject<\/a>.<\/p>\n<\/p><\/div>\n<div class=\"mt24\"><\/div>\n<\/div>\n<p>            <span class=\"d-none\" itemprop=\"commentCount\">42<\/span> <\/p><\/div>\n<\/div>\n<p>[ad_2]<\/p>\n<p>solved Why can templates only be implemented in the header file? <\/p>\n","protected":false},"excerpt":{"rendered":"<p>[ad_1] Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer. Anyway, the reason your code is failing is that, when instantiating a template, the compiler creates a new class with the given template argument. For example: template&lt;typename T&gt; struct Foo { &#8230; <a title=\"(Solved) Why can templates only be implemented in the header file?\" class=\"read-more\" href=\"https:\/\/jassweb.com\/solved\/solved-why-can-templates-only-be-implemented-in-the-header-file\/\" aria-label=\"More on (Solved) Why can templates only be implemented in the header file?\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,364,435,373],"class_list":["post-3884","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-c-faq","tag-templates","tag-undefined-reference"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>(Solved) Why can templates only be implemented in the header file? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-why-can-templates-only-be-implemented-in-the-header-file\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"(Solved) Why can templates only be implemented in the header file? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer. 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