{"id":6151,"date":"2021-06-04T07:06:51","date_gmt":"2021-06-04T07:06:51","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/alias-templates-and-template-parameters\/"},"modified":"2021-06-04T07:06:51","modified_gmt":"2021-06-04T07:06:51","slug":"alias-templates-and-template-parameters","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/alias-templates-and-template-parameters\/","title":{"rendered":"Alias Templates and Template Parameters"},"content":{"rendered":"

Today, I write about two topics: alias templates and template parameters. Alias templates are a way to give a name to a family of types. Template parameters can be types, non-types, and templates themselves.<\/p>\n

<\/p>\n

 \"templatesParameters\"<\/p>\n

Let’s start with the alias templates.<\/p>\n

Alias Templates<\/h2>\n

With C++11, we got alias templates. Alias templates provide a means to give a convenient name to a family of types. The following code snippet presents the idea for the class template Matrix.<\/p>\n

<\/p>\n

\n
template<\/span> <<\/span>typename<\/span> T, int<\/span> Line, int<\/span> Col><\/span>\r\nclass<\/span> Matrix<\/span>{\r\n    ....\r\n};\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Matrix<\/code> has three template parameters. The type parameter, and the non-type parameters, and Col<\/code> (I will write about template parameters in the next section.)<\/p>\n
For readability, I want to have two special matrices: a Square<\/code> and a Vector<\/code>. A Square<\/code>‘s number of lines and columns should be equal. A Vector<\/code>‘s line size should be one. Thanks to type aliases, I can express my ideas directly in code.<\/p>\n
<\/p>\n
\n
template<\/span> <<\/span>typename<\/span> T, int<\/span> Line><\/span>\r\nusing<\/span> Square =<\/span> Matrix<<\/span>T, Line, Line><\/span>; \/\/ (1)<\/span>\r\n\r\ntemplate<\/span> <<\/span>typename<\/span> T, int<\/span> Line><\/span>\r\nusing<\/span> Vector =<\/span> Matrix<<\/span>T, Line, 1<\/span>><\/span>;    \/\/ (2)<\/span>\r\n<\/pre>\n<\/div>\n
 <\/p>\n
The keyword using <\/code>((1) and (2)) declares a type alias. While the primary template Matrix<\/code> can be parametrized in the three dimensions T<\/code>, Line<\/code>, and Col<\/code>, the type aliases Square<\/code> and Vector <\/code>reduce the parametrization to the two dimensions T<\/code> and Line<\/code>. From this point of view, alias templates enable it to create intuitive names for partially bound templates. Using Square<\/code> and Vector<\/code> is straightforward.<\/p>\n
<\/p>\n
\n
Matrix<<\/span>int<\/span>, 5<\/span>, 3<\/span>><\/span> ma;\r\nSquare<<\/span>double<\/span>, 4<\/span>><\/span> sq;\r\nVector<<\/span>char<\/span>, 5<\/span>><\/span> vec;\r\n<\/pre>\n<\/div>\n
 <\/p>\n
An excellent use case of alias templates is the type-traits library<\/a>.<\/p>\n
Type-Traits Library<\/h3>\n
When you apply std::move(arg)<\/code> on a value arg<\/code>, the compiler uses typically std::remove_reference<\/code><\/a> to remove a reference from the underlying type:<\/p>\n
<\/p>\n
\n
static_cast<\/span><<\/span>std::<\/span>remove_reference<<\/span>decltype(arg)>::<\/span>type&&><\/span>(arg);   \/\/ (1)<\/span>\r\n\r\nstatic_cast<\/span><<\/span>std::<\/span>remove_reference_t<\/span><<\/span>decltype(arg)>&&><\/span>(arg);       \/\/ (2)<\/span>\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Thanks to alias templates, version (line 2) have been valid since C++14. The following helper type is available:<\/p>\n
<\/p>\n
\n
template<\/span><<\/span> class<\/span> T<\/span> ><\/span>\r\nusing<\/span> remove_reference_t<\/span> =<\/span> typename<\/span> remove_reference<<\/span>T>::<\/span>type;\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Of course, the corresponding helper types for the other functions of the type-traits library<\/a> returning a type are also available with C++14.<\/p>\n
 <\/p>\n
The previously defined class template Matrix<\/code> uses the two non-type template parameters Line<\/code> and Col<\/code>.<\/p>\n<\/p>\n
Template Parameters<\/h2>\n
 Template parameters can be types, non-types, and templates themselves.<\/p>\n
Types<\/h3>\n
Okay, types are the most often used template parameters. Here are a few examples:<\/p>\n
\n
std::<\/span>vector<<\/span>int<\/span>><\/span> myVec;\r\nstd::<\/span>map<<\/span>std::<\/span>string, int<\/span>><\/span> myMap;\r\nstd::<\/span>lock_guard<<\/span>std::<\/span>mutex><\/span> myLockGuard;\r\n<\/pre>\n<\/div>\n
Non-Types<\/h3>\n
Non-types can be a<\/p>\n