![\"\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2024\/10\/Contracts-1030x505.png\")
<\/figure>\n\n\n\nContracts should already be part of C++20 but were removed in the standard meeting in Cologne. Here is what Herb Sutter said about contracts on Sutter\u2019s Mill<\/a>: \u201ccontracts is the most impactful feature of C++20 so far, and arguably the most impactful feature we have added to C++ since C++11<\/em>.\u201d. With C++26, we probably get them.<\/p>\n\n\n\n This post is based on the proposal P2961R2<\/a>.<\/p>\n\n\n\n First of all.<\/p>\n\n\n\n A contract specifies interfaces for software components in a precise and checkable way. These software components are functions and methods that must fulfill preconditions, postconditions, and invariants. Here are the definitions:<\/p>\n\n\n\n The precondition and the postcondition are placed outside the function definition, but the invariant is placed inside the function definition. A predicate is an expression that returns a boolean.<\/p>\n\n\n\n Before I show you the first example, let me write about the contract design goals.<\/p>\n\n\n\n You may wonder why the assertion has such a long keyword.<\/p>\n\n\n\n The ideal keyword for the assertion would be The break of the contract causes a runtime error.<\/p>\n\n\n\n My next post will continue with the minor C++26 core language features.<\/p>\n","protected":false},"excerpt":{"rendered":" Contracts allow you to specify preconditions, postconditions, and invariants for functions. Contracts should already be part of C++20 but were removed in the standard meeting in Cologne. Here is what Herb Sutter said about contracts on Sutter\u2019s Mill: \u201ccontracts is the most impactful feature of C++20 so far, and arguably the most impactful feature we […]<\/p>\n","protected":false},"author":21,"featured_media":10185,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[559],"tags":[481],"class_list":["post-10182","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c26-blog","tag-contracts"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10182","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=10182"}],"version-history":[{"count":23,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10182\/revisions"}],"predecessor-version":[{"id":10855,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10182\/revisions\/10855"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/10185"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=10182"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=10182"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=10182"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}What is a Contract?<\/h2>\n\n\n\n
\n
Design Goals <\/h2>\n\n\n\n
\n
Now comes the first example.<\/p>\n\n\n\nFirst example<\/h2>\n\n\n\n
int<\/span> f<\/span>(int<\/span> i)\n pre (i >=<\/span> 0<\/span>)\n post (r:<\/span> r ><\/span> 0<\/span>)\n{\n contract_assert (i >=<\/span> 0<\/span>);\n return<\/span> i+<\/span>1<\/span>;\n}\n<\/pre><\/div>\n\n\n\npre<\/code> and post<\/code><\/h3>\n\n\n\n\n
post<\/code><\/h3>\n\n\n\n\n
contract_assert<\/code><\/h3>\n\n\n\n\n
The
assert <\/code>Issue<\/h2>\n\n\n\nassert <\/code>but not contract_assert<\/code>. assert <\/code>is used in most programming languages to express contract-like assertions. But C++ has a legacy issue.<\/p>\n\n\n\n#include <cassert><\/span>\n\nvoid<\/span> f<\/span>() {\n int<\/span> i =<\/span> get_i();\n assert(i >=<\/span> 0<\/span>); \/\/ identical syntax for contract assert and macro assert!<\/span>\n use_i(i);\n}\n<\/pre><\/div>\n\n\n\nassert<\/code> is already a macro from the header <cassert><\/code>.<\/p>\n\n\n\nBreak Of Contract<\/h2>\n\n\n\n
\/\/ contract.cpp<\/span>\n\n#include <iostream><\/span>\n\nint<\/span> f<\/span>(int<\/span> i)\n pre (i >=<\/span> 0<\/span>)\n post (r:<\/span> r ><\/span> 0<\/span>)\n{\n contract_assert (i >=<\/span> 0<\/span>);\n return<\/span> i+<\/span>1<\/span>;\n}\n\nint<\/span> main<\/span>() {\n\n std::<\/span>cout <<<\/span> '\\n'<\/span>; \n \n f(-<\/span>1<\/span>);\n \n std::<\/span>cout <<<\/span> '\\n'<\/span>;\n \n}\n<\/pre><\/div>\n\n\n\n![\"\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2024\/10\/Contract.png\")
<\/figure>\n\n\n\nWhat’s Next<\/h2>\n\n\n\n