{"id":5940,"date":"2020-07-09T05:44:24","date_gmt":"2020-07-09T05:44:24","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/c-20-consteval-and-constinit\/"},"modified":"2023-06-26T09:47:46","modified_gmt":"2023-06-26T09:47:46","slug":"c-20-consteval-and-constinit","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/c-20-consteval-and-constinit\/","title":{"rendered":"Two new Keywords in C++20: consteval and constinit"},"content":{"rendered":"

With C++20, we get two new keywords: consteval<\/span> and constinit<\/span>. consteval<\/span> produces a function executed at compile-time, and constinit<\/span> guarantees that a variable is initialized at compile-time.<\/p>\n

<\/p>\n

 \"TimelineCpp20\"<\/p>\n

When you read my previous short description of consteval<\/span> and constinit<\/span> you may have the impression that both specifiers are pretty similar to constexpr<\/span>. To make it short, you are right. Before I compare the keywords consteval<\/span>, constinit<\/span>, constexpr<\/span>, and good old const<\/span>, I have to introduce the new specifiers consteval<\/span> and constinit<\/span>.<\/p>\n

consteval<\/span><\/h2>\n
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consteval int<\/span> sqr<\/span>(int<\/span> n) {\r\n    return<\/span> n * <\/span>n;\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
consteval<\/span> creates a so-called immediate function. Each invocation of an immediate function creates a compile-time constant. To say it more directly. A consteval<\/span> (immediate) function is executed at compile-time.<\/p>\n
consteval<\/code> cannot be applied to destructors or functions which allocate or deallocate. You can only use at most one of consteval<\/span>, constexpr<\/span>, or constinit<\/span> specifiers in a declaration. An immediate function (consteval<\/span>) is implicit inline<\/span> and must fulfill a constexpr function’s requirements.<\/p>\n
The requirements for a constexpr<\/span> function in C++14 and, therefore, a consteval<\/span> function is:<\/p>\n
A constexpr function<\/span> can<\/p>\n