Last July we agreed to remove the convention that declarations of POD classes use the class-key struct (and others class). Attached is a patch to update the GCC Coding Conventions and the rationale to reflect the decision. I replaced it with a mild suggestion to use struct for C structs and class for all others, that I Jason and Jonathan preferred but tried to make it clear it wasn't a hard and fast rule.
Martin
PR 61339 - mismatch between struct and class [-Wmismatched-tags] ChangeLog: * htdocs/codingconventions.html (Struct Definitions): Remove old convention. (Class Definitions): Same. * htdocs/codingrationale.html (Struct Definitions): Document convention change. diff --git a/htdocs/codingconventions.html b/htdocs/codingconventions.html index 03a77063..f4732ef6 100644 --- a/htdocs/codingconventions.html +++ b/htdocs/codingconventions.html @@ -888,9 +888,20 @@ Variables may be simultaneously defined and tested in control expressions. <h4 id="Struct_Use">Struct Definitions</h4> <p> -Use the <code>struct</code> keyword for -<a href="https://en.wikipedia.org/wiki/Plain_old_data_structure";> -plain old data (POD)</a> types. +Some coding conventions, including GCC's own in the past, recommend +using the <code>struct</code> keyword (also known as the <i>class-key</i>) +for <a href="https://en.wikipedia.org/wiki/Plain_old_data_structure";> +plain old data (POD)</a> types. However, since the POD concept has been +replaced in C++ by a set of much more nuanced distinctions, the current +guidance (though not a requirement) is to use the <code>struct</code> +<i>class-key</i> when defining structures that could be used without +change in C, and use <code>class</code> for all other classes. It is +recommended to use the same <i>class-key</i> consistently in all +declarations and, if necessary, in uses of the class. +The <code>-Wmismatched-tags</code> warning option helps detect mismatches. +The <code>-Wredundant-tags</code> GCC option further helps identify places +where the <i>class-key</i> can safely be omitted. + </p> <p> @@ -900,14 +911,13 @@ plain old data (POD)</a> types. <h4 id="Class_Use">Class Definitions</h4> <p> -Use the <code>class</code> keyword for -<a href="https://en.wikipedia.org/wiki/Plain_old_data_structure";> -non-POD</a> types. +See the guidance in <a href="#Struct_Use">Struct Definitions</a> for +the suggested choice of a <i>class-key</i>. </p> <p> -A non-POD type will often (but not always) -have a declaration of a +A class defined with the class-key <code>class</code> type will often +(but not always) ave a declaration of a <a href="https://en.wikipedia.org/wiki/Special_member_functions";> special member function</a>. If any one of these is declared, diff --git a/htdocs/codingrationale.html b/htdocs/codingrationale.html index a2618b64..0b44f1da 100644 --- a/htdocs/codingrationale.html +++ b/htdocs/codingrationale.html @@ -54,15 +54,16 @@ if (info *q = get_any_available_info ()) { <h4 id="struct">Struct Definitions</h4> <p> -In the C++ standard, -structs and classes differ only in the default access rules. -We prefer to use these two keywords to signal more information. -</p> - -<p> -Note that under this definition, -structs may have member functions. -This freedom is useful for transitional code. +In the C++ standard, structs and classes differ only in the default +access rules. In the past, there was a mild preference among some +GCC developers for using these two keywords to indicate whether or +not a class met the criteria for a Plain Old Data (POD) type. +However, this convention was never consistently adhered to or fully +socialized. A review of a patch to +<a href="https://gcc.gnu.org/pipermail/gcc-patches/2019-July/525526.html";> +add support for POD struct convention (PR 61339)</a> revealed that +the convention lacked broad enough support within the GCC developer +community. As a result, the convention was removed. </p>
