{"id":6160,"date":"2021-06-07T21:13:00","date_gmt":"2021-06-07T21:13:00","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/template-arguments\/"},"modified":"2021-06-07T21:13:00","modified_gmt":"2021-06-07T21:13:00","slug":"template-arguments","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/template-arguments\/","title":{"rendered":"Template Arguments"},"content":{"rendered":"

It is pretty interesting how the compiler deduces the types for the template arguments. To make it short, you get most of the time the type you expect. The rules do not only apply to function templates (C++98) but also to auto<\/code> (C++11), to class templates (C++17), and concepts (C++20).<\/p>\n

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\"templatesArguments\"<\/p>\n

C++ has supported function template argument deduction since its beginning. Here is a short recap.<\/p>\n

Function Template Argument Deduction<\/h2>\n

Let me invoke a function template max<\/code> for int <\/code>and double<\/p>\n

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template<\/span> <<\/span>typename<\/span> T><\/span>\r\nT max(T lhs, T rhs) {\r\n    return<\/span> (lhs ><\/span> rhs)?<\/span> lhs :<\/span> rhs;\r\n}\r\n\r\nint<\/span> main() {\r\n  \r\n    max(10<\/span>, 5<\/span>);         \/\/ (1)<\/span>\r\n    max(10.5<\/span>, 5.5<\/span>);     \/\/ (2)<\/span>\r\n  \r\n}\r\n<\/pre>\n<\/div>\n
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In this case, the compiler deduces the template arguments from the function arguments. C++ Insights<\/a> shows that the compiler creates a fully specialized function template for max<\/code> for int<\/code> (1) and for double<\/code> (2).<\/p>\n
\"functionTempateIntDouble\"<\/p>\n
The process of template type deduction, such as in this case, produces most of the time the expected type. It is pretty enlightening to analyze this process deeper.<\/p>\n<\/p>\n
Template Type Deduction<\/h2>\n
When deducing the template type, three entities come into play: T, ParameterType, and expression.<\/p>\n
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template<\/span> <<\/span>typename<\/span> T><\/span>\r\nvoid<\/span> func(ParameterType param);\r\n\r\nfunc(expression);\r\n<\/pre>\n<\/div>\n
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Two types are deduced:<\/p>\n