blocks|key|2927480|text|这种(符合标准的)歧义的根本原因似乎在于转换的成本:重载解析试图最小化将参数转换为相应参数所执行的操作。另一个例子：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2927481|void+g()+{}

void+f(void(*)())+{}
void+f(void(&)())+{}

int+main()+{
++++f(g);+//+Ambiguous
}|code-block|syntax|javascript|2927482|2927483|以下是必须遵循的标准。不是某些函数模板的特殊化的函数比那些两者都是同样好的匹配的函数更好(参见over.match.best/(1.3)，(1.6))。在我们的例子中，执行的转换是数组到指针的转换，这是一个具有精确匹配秩的Lvalue转换(根据over.ics.user中的表12+)。Over.ics.ank/3：|2927484|2927485|blockquote|2927486|2927487|2927488|标准转换序列S1是比标准转换序列S2更好的转换序列，如果|unordered-list-item|offset|length|style|CODE|2927489|2927490|-+`S1`+is+a+proper+subsequence+of+`S2`+(comparing+the+conversion+sequences+in+the+canonical+form++defined+by+13.3.3.1.1,+**excluding+++any+Lvalue+Transformation**;+the+identity+conversion+sequence+is+considered+to+be+a+subsequence+of+any+non-identity+conversion+++sequence)+or,+if+not+that,
-+the+rank+of+`S1`+is+better+than+the+rank+of+`S2`,+or+`S1`+and+`S2`+have+the+same+rank+and+are+distinguishable+by+the+rules+in+the+paragraph+below,+or,+if+not+that,
-+[..]|2927491|​|2927492|第一个项目符号排除了我们的转换(因为它是一个左值转换)。第二个需要不同的等级，这是不存在的，因为两个转换都有精确匹配的等级；“下面段落中的规则”，即over.ics.ank/4，也不包括数组到指针的转换。|2927493|因此，信不信由你，这两个转换序列中没有一个比另一个更好，因此选择了char+const*-overload。|2927494|2927495|2927496|template+<typename+T>
auto+foo(T+s)
++->+std::enable_if_t<std::is_convertible<T,+char+const*>{}>
{
++++std::cout+<<+"raw,+size="+<<+std::strlen(s)+<<+std::endl;
}|2927497|。|2927498|entityMap^0|0|0|0|0|0|0|0|0|6|2|G|2|0|0|0|0|0|X|B|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|1F|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|K|3|-4|5|L|7|1H|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|O|3|P|5|Q|7|1K|8|@$R|1L|S|1M|T|U]|$R|1N|S|1O|T|U]]|9|@]|A|$]]|$1|V|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]|$1|W|3|X|5|D|7|1Q|8|@]|9|@]|A|$E|F]]|$1|Y|3|Z|5|6|7|1R|8|@]|9|@]|A|$]]|$1|10|3|11|5|6|7|1S|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|1T|8|@$R|1U|S|1V|T|U]]|9|@]|A|$]]|$1|14|3|-4|5|6|7|1W|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|1X|8|@]|9|@]|A|$]]|$1|16|3|17|5|D|7|1Y|8|@]|9|@]|A|$E|F]]|$1|18|3|19|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|1A|3|-4|5|6|7|20|8|@]|9|@]|A|$]]]|1B|$]]

The fundamental reason for this (standard-conforming) ambiguity appears to lie within the cost of conversion: Overload resolution tries to minimize the operations performed to convert an argument to the corresponding parameter. An array is effectively the pointer to its first element though, decorated with some compile-time type information. An array-to-pointer conversion doesn't cost more than e.g. saving the address of the array itself, or initializing a reference to it. From that perspective, the ambiguity seems justified, although conceptually it is unintuitive (and may be subpar). In fact, this argumentation applies to all Lvalue Transformations, as suggested by the quote below. Another example:

<pre><code>void g() {}

void f(void(*)()) {}
void f(void(&amp;)()) {}

int main() {
 f(g); // Ambiguous
}
</code></pre>

<hr>

The following is obligatory standardese. Functions that are not specializations of some function template are preferred over ones that are if both are otherwise an equally good match (see [over.match.best]/(1.3), (1.6)). In our case, the conversion performed is an array-to-pointer conversion, which is an Lvalue Transformation with Exact Match rank (according to table 12 in [over.ics.user]). [over.ics.rank]/3:

<blockquote>
 <ul>
 <li>Standard conversion sequence <code>S1</code> is a better conversion sequence than standard conversion sequence <code>S2</code> if
 
 <ul>
 <li><code>S1</code> is a proper subsequence of <code>S2</code> (comparing the conversion sequences in the canonical form defined by 13.3.3.1.1, excluding
 any Lvalue Transformation; the identity conversion sequence is considered to be a subsequence of any non-identity conversion
 sequence) or, if not that,</li>
 <li>the rank of <code>S1</code> is better than the rank of <code>S2</code>, or <code>S1</code> and <code>S2</code> have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,</li>
 <li>[..]</li>
 </ul></li>
 </ul>
</blockquote>

The first bullet point excludes our conversion (as it is an Lvalue Transformation). The second one requires a difference in ranks, which isn't present, as both conversions have Exact match rank; The "rules in the paragraph below", i.e. in [over.ics.rank]/4, don't cover array-to-pointer conversions either. 
So believe it or not, none of both conversion sequences is better than the other, and thus the <code>char const*</code>-overload is picked.

<hr>

Possible workaround: Define the second overload as a function template as well, then partial ordering kicks in and selects the first one.

<pre><code>template &lt;typename T&gt;
auto foo(T s)
 -&gt; std::enable_if_t&lt;std::is_convertible&lt;T, char const*&gt;{}&gt;
{
 std::cout &lt;&lt; "raw, size=" &lt;&lt; std::strlen(s) &lt;&lt; std::endl;
}
</code></pre>

<a href="http://coliru.stacked-crooked.com/a/ef69810b39083a6c">Demo</a>.

Let's say I'm writing a function to print the length of a string:

<pre><code>template &lt;size_t N&gt;
void foo(const char (&amp;s)[N]) {
 std::cout &lt;&lt; "array, size=" &lt;&lt; N-1 &lt;&lt; std::endl;
}

foo("hello") // prints array, size=5
</code></pre>

Now I want to extend <code>foo</code> to support non-arrays:

<pre><code>void foo(const char* s) {
 std::cout &lt;&lt; "raw, size=" &lt;&lt; strlen(s) &lt;&lt; std::endl;
}
</code></pre>

But it turns out that this breaks my original intended usage:

<pre><code>foo("hello") // now prints raw, size=5
</code></pre>

Why? Wouldn't that require an array-to-pointer conversion, whereas the template would be an exact match? Is there a way to ensure that my array function gets called?

Why does pointer decay take priority over a deduced template?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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假设我正在编写一个函数来打印字符串的长度：template <size_t N>void foo(const char (&s)[N]) { std::cout << "array, size=" << N-1 << std::endl;}foo("hello") // prints array, size=5现...

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