blocks|key|2285139|text|你可以这样写它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2285140|template+<typename+T1,+typename+T2>
++++requires+AreEqComparable<T1,+T2>
bool+are_equal(T1+a,+T2+b)
{
++++//+...
}|code-block|syntax|javascript|2285141|这里，我们使用一个requires-子句对类型模板参数施加一个要求。|2285142|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You can write it like this:

<pre><code>template &lt;typename T1, typename T2&gt;
 requires AreEqComparable&lt;T1, T2&gt;
bool are_equal(T1 a, T2 b)
{
 // ...
}
</code></pre>

Here, we use a requires-clause to impose a requirement on the type template parameters.

blocks|key|2497702|text|你可以这样写：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2497703|template+<typename+T1,+AreEqComparable<T1>+T2>
bool+are_equal(T1,+T2);|code-block|syntax|javascript|2497704|这相当于：|2497705|template+<typename+T1,+typename+T2>
++++requires+AreEqComparable<T2,+T1>
bool+are_equal(T1,+T2);|2497706|在这里，类型在约束中翻转，是AreEqComparable<T2,+T1>而不是AreEqComparable<T1,+T2>。这对许多概念来说肯定很重要，但可能不是这个，因为==本身在C%2B%2B20中是对称的(除了不应该存在于实际代码中的病理情况)。如果你真的想确定这种对称性是有效的，你总是可以在概念中明确它(就像EqualityComparableWith在工作草案中一样)：|offset|length|style|CODE|2497707|template<typename+T1,+typename+T2>
concept+AreEqComparable+=+requires(T1+a,+T2+b)+{
++++{+a+==+b+}+->+bool;
++++{+b+==+a+}+->+bool;
};|2497708|2497709|实际上，您可以通过颠倒模板参数的顺序(h/t+Matthieu+M)，以正确的顺序获得所需的约束：|2497710|template+<typename+T2,+AreEqComparable<T2>+T1>
bool+are_equal(T1,+T2);|2497711|entityMap|0|LINK|mutability|MUTABLE|url|http://eel.is/c%2B%2Bdraft/concept.equalitycomparable^0|0|0|0|0|E|N|14|N|2G|2|4D|M|4D|M|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|15|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|17|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|18|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|19|8|@$M|1A|N|1B|O|P]|$M|1C|N|1D|O|P]|$M|1E|N|1F|O|P]|$M|1G|N|1H|O|P]]|9|@$M|1I|N|1J|1|1K]]|A|$]]|$1|Q|3|R|5|D|7|1L|8|@]|9|@]|A|$E|F]]|$1|S|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|1N|8|@]|9|@]|A|$]]|$1|V|3|W|5|D|7|1O|8|@]|9|@]|A|$E|F]]|$1|X|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14]]]]

You can write:

<pre><code>template &lt;typename T1, AreEqComparable&lt;T1&gt; T2&gt;
bool are_equal(T1, T2);
</code></pre>

This is equivalent to:

<pre><code>template &lt;typename T1, typename T2&gt;
 requires AreEqComparable&lt;T2, T1&gt;
bool are_equal(T1, T2);
</code></pre>

The types are flipped in the constraint here, <code>AreEqComparable&lt;T2, T1&gt;</code> instead of <code>AreEqComparable&lt;T1, T2&gt;</code>. This will certainly matter for many concepts, but probably not this one in particular since <code>==</code> itself becomes symmetric in C++20 (short of pathological cases which should not exist in real code). And if you want to be really sure that this symmetry is valid, you can always make it explicit in the concept (as <a href="http://eel.is/c++draft/concept.equalitycomparable" rel="noreferrer"><code>EqualityComparableWith</code></a> is in the working draft):

<pre><code>template&lt;typename T1, typename T2&gt;
concept AreEqComparable = requires(T1 a, T2 b) {
 { a == b } -&gt; bool;
 { b == a } -&gt; bool;
};
</code></pre>

<hr />

You can actually get the constraint you want in the correct order by flipping the order of the template parameters (h/t Matthieu M.):

<pre><code>template &lt;typename T2, AreEqComparable&lt;T2&gt; T1&gt;
bool are_equal(T1, T2);
</code></pre>

blocks|key|2497730|text|还有一种完全避免引入模板参数的语法(以添加其他冗余为代价)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2497731|bool+are_equal(auto+x,auto+y)
++requires+AreEqComparable<decltype(x),decltype(y)>
++{return+x==y;}|code-block|syntax|javascript|2497732|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Yet another syntax that avoids introducing template parameters at all (at the cost of adding other redundancy):

<pre><code>bool are_equal(auto x,auto y)
 requires AreEqComparable&lt;decltype(x),decltype(y)&gt;
 {return x==y;}
</code></pre>

blocks|key|2498057|text|在GCC+8.2.0中，概念应该写成这样：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2498058|concept+bool+ConceptName+=+/*+...+*/|code-block|syntax|javascript|2498059|但是C%2B%2B+Templates:+The+Complete+Guide没有提到bool。由于C%2B%2B20标准没有发布，很难说哪种标准是正确的。|offset|length|style|CODE|2498060|对于需要一个参数(不一定是类型)的概念，有一个简写：|2498061|template+<UnaryConceptName+T>|2498062|对于那些需要两个或更多参数的人，没有速记：|2498063|template+<typename+T1,+typename+T2>+requires+BinaryConceptName<T1,+T2>|2498064|typename可以替换为特定的类型名称。|2498065|顺便说一下：|2498066|2498067|我上面提到的那本书对概念进行了简要的介绍。|ordered-list-item|2498068|2498069|2498070|在GCC中使用-fconcepts来启用概念。|2498071|2498072|entityMap|0|LINK|mutability|MUTABLE|url|http://www.tmplbook.com/^0|0|0|13|4|2|X|0|0|0|0|0|0|0|8|0|0|0|0|0|0|7|A|0|0^^$0|@$1|2|3|4|5|6|7|1F|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1G|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1H|8|@$I|1I|J|1J|K|L]]|9|@$I|1K|J|1L|1|1M]]|A|$]]|$1|M|3|N|5|6|7|1N|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|1O|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|1P|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1Q|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1R|8|@$I|1S|J|1T|K|L]]|9|@]|A|$]]|$1|W|3|X|5|6|7|1U|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]|$1|Z|3|10|5|11|7|1W|8|@]|9|@]|A|$]]|$1|12|3|-4|5|6|7|1X|8|@]|9|@]|A|$]]|$1|13|3|-4|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|14|3|15|5|11|7|1Z|8|@$I|20|J|21|K|L]]|9|@]|A|$]]|$1|16|3|-4|5|6|7|22|8|@]|9|@]|A|$]]|$1|17|3|-4|5|6|7|23|8|@]|9|@]|A|$]]]|18|$19|$5|1A|1B|1C|A|$1D|1E]]]]

In GCC 8.2.0, concepts should be written like:
<pre><code>concept bool ConceptName = /* ... */
</code></pre>
But <a href="http://www.tmplbook.com/" rel="nofollow noreferrer">C++ Templates: The Complete Guide</a> doesn't mention the <code>bool</code>. Since the C++20 standard is not released, it's hard to say which is right.
For concepts that need one parameter (not necessarily type), there's a shorthand:
<pre><code>template &lt;UnaryConceptName T&gt;
</code></pre>
For those that need two or more parameters, there's no shorthand:
<pre><code>template &lt;typename T1, typename T2&gt; requires BinaryConceptName&lt;T1, T2&gt;
</code></pre>
<code>typename</code> can be replaced with specific type name.
By the way:
<ol>
<li>The book I mentioned above gives a brief introduction to concepts.
</li>
<li>use <code>-fconcepts</code> in GCC to enable concepts.
</li>
</ol>

It is confirmed that in the upcoming <code>c++20</code> standard, according to <a href="https://www.reddit.com/r/cpp/comments/cfk9de/201907_cologne_iso_c_committee_trip_report_the/" rel="noreferrer">this reddit report</a> from the recent <a href="https://isocpp.org/std/meetings-and-participation/upcoming-meetings" rel="noreferrer">Cologne ISO C++ Meeting</a>, we will be able to specify a template's <a href="https://en.wikipedia.org/wiki/Concepts_(C%2B%2B)" rel="noreferrer">concept</a> and for each class/function template, we will be able to set the <a href="https://en.cppreference.com/w/cpp/language/constraints" rel="noreferrer">constraints</a> on its types. However, in documentations and tutorials (e.g. <a href="https://en.cppreference.com/w/cpp/language/constraints" rel="noreferrer">here</a>), I could not find the correct syntax for the multi-type use-case.
<hr />
Suppose we have a multi-type concept:
<pre class="lang-cpp prettyprint-override"><code>template&lt;typename T1, typename T2&gt;
concept AreEqComparable = requires(T1 a, T2 b) {
 { a == b } -&gt; bool;
};
</code></pre>
Let's say, I want to define a simple comparison function between two different types. How can I do that? More specifically, what should I write in the <code>???</code> part of the code below:
<pre class="lang-cpp prettyprint-override"><code>???
bool are_equal(T1 a, T2 b) { return a == b; }
</code></pre>
I couldn't find any reference to this case in <a href="https://en.cppreference.com/w/cpp/language/constraints" rel="noreferrer">here</a>, <a href="https://cppdepend.com/blog/?p=524" rel="noreferrer">here</a>, and even <a href="https://en.wikipedia.org/wiki/Concepts_(C%2B%2B)" rel="noreferrer">here</a>. I have randomly tried something like:
<pre><code>/* 1 */ template&lt;AreEqComparable T1, T2&gt;
/* 2 */ AreEqComparable&lt;T1, T2&gt;
/* 3 */ template&lt;AreEqComparable&lt;T1, T2&gt;&gt;
</code></pre>
But all of them throw syntax errors. I think the answer should lie somewhere in the specification <a href="http://www.stroustrup.com/good_concepts.pdf" rel="noreferrer">P0557</a> by <a href="https://en.wikipedia.org/wiki/Bjarne_Stroustrup" rel="noreferrer">Bjarne Stroustrup</a>, but I wasn't able to find it after a quick look.

C++20: Concepts of multiple types and its constraint, correct syntax?

已经确认，在即将到来的c++20标准中，根据最近中的，我们将能够指定模板的，并且对于每个类/函数模板，我们将能够在其类型上设置。然而，在文档和教程(例如)中，我找不到适用于多类型用例的正确语法。假设我们有一个多类型的概念：template<typename T1, typename T2>concept AreEqCo...

问C++20:多类型的概念及其约束，正确的语法？
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