blocks|key|3243105|text|这意味着你不能做下面这样的事情|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3243106|++void+foo()
++{
+++++++template+<typename+T>+//Error
+++++++T+something;
++}|code-block|syntax|javascript|3243107|仅允许在全局、命名空间或类范围内进行模板声明。:)|3243108|3243109|它背后的原因是什么？|blockquote|3243110|3243111|3243112|这是不允许的，因为标准是这样说的。|3243113|ISO%2B%2B-98(14.2节)|3243114|3243115|模板声明只能作为命名空间或类范围声明出现。|3243116|3243117|3243118|这有意义吗？|3243119|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|13|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|14|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|15|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|16|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|17|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|18|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|T|3|U|5|L|7|1B|8|@]|9|@]|A|$]]|$1|V|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|1E|8|@]|9|@]|A|$]]|$1|Z|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|10|$]]

It means you cannot do something like the following

<pre><code> void foo()
 {
 template &lt;typename T&gt; //Error
 T something;
 }
</code></pre>

Template declarations are only permitted at global, namespace, or class scope. :)

<blockquote>
 What is the reasoning behind it?
</blockquote>

It is not allowed because the Standard says so . 

ISO C++-98 (Section 14.2) 

<blockquote>
 A template declaration can appear only as a namespace or class scope declaration.
</blockquote>

Does that make sense?

blocks|key|300969|text|这到底有什么用呢？所以你可以声明只能在函数中使用的模板变量？这真的有用吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|300970|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

What exactly would be the use? So you can declare template variables that you can only use within the function? Is that really useful?

blocks|key|3243199|text|简而言之，这是因为编写c/c%2B%2B编译器和标准的人希望它是这样的。函数中的模板必须被认为太混乱和/或难以理解或解析，所以他们禁止这样做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3243200|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The short answer to why this is, is because that how the guys who wrote the c/c++ compilers and standards wanted it to be. Templates inside functions must have been deemed too chaotic and/or difficult to understand or parse, so they forbade it.

blocks|key|303168|text|只有当你用一个函数创建了具有不同类型的模板的多个实例时，这才是有用的。无论如何，将你的私有类移出你的函数。如果这开始弄乱你的类，那么它们就太大了，需要重构。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|303169|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The only time this would be useful would be if you created multiple instances of the template with different types withing the one function. Move your private classes out of your functions anyway. If that starts to clutter up your classes then they are too big and need to be refactored.

blocks|key|300336|text|我的猜测是它很难实现，这就是为什么它是不被允许的(在标准C%2B%2B03中)。另一方面，在函数之外编写类模板是一种可以接受的解决方案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|300337|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

My guess is that it is hard to implement, that's why it is not allowed (in Standard C++03). Writing class templates outside of functions is an acceptable solution from the other hand.

blocks|key|300383|text|问题可能与模板实现的历史方式有关:早期的实现技术(以及一些至今仍在使用的技术)要求模板中的所有符号都具有外部链接。(实例化是通过在单独的文件中生成等效代码来完成的。)在函数内部定义的名称永远不会有链接，并且不能在定义它们的作用域之外引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|300384|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The problem is probably linked to the historical way templates were implemented: early implementation techniques (and some still used today) require all symbols in a template to have external linkage. (Instantiation is done by generating the equivalent code in a separate file.) And names defined inside a function never have linkage, and cannot be referred to outside of the scope in which they were defined.

blocks|key|300391|text|答案“因为标准这么说”，当然是正确的，但让我们考虑一下通用的lambdas。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|300392|在C%2B%2B14和C%2B%2B17中，据我所知，泛型lambda是编写类似模板的代码的唯一方法：|300393|++++auto+lambda+=+[](auto+x)+{+};
++++lambda.operator()<int>(0);|code-block|syntax|javascript|300394|从技术上讲，你可以用它来编写任何类型的模板代码。不过，您必须努力绕过此方法的各种限制。|300395|不过，在C%2B%2B20中这会更简单。使用泛型lambdas中的模板参数列表，您将能够编写如下代码：|300396|++++auto+size+=+[]<class+T>()+{+return+sizeof(T);+};
++++static_assert(4+==+size.operator()<int>());|300397|GCC已经支持这种语法。|300398|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|X|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

The answer "because standard says so", is of course correct, but let's consider generic lambdas.

In C++14 and C++17 generic lambdas are the only way of writing template-like code that I know of:

<pre><code> auto lambda = [](auto x) { };
 lambda.operator()&lt;int&gt;(0);
</code></pre>

Technically, you can write any kind of template code just with that. Though you'll have to work hard to work around various limitations of this approach. 

That will be simpler in C++20 though. With template parameter list in generic lambdas you will be able to write code like this:

<pre><code> auto size = []&lt;class T&gt;() { return sizeof(T); };
 static_assert(4 == size.operator()&lt;int&gt;());
</code></pre>

GCC already supports this syntax.

Reading <a href="http://www.amazon.co.uk/Templates-Complete-Guide-David-Vandevoorde/dp/0201734842" rel="noreferrer">C++ Templates: The Complete Guide</a> and it says 

<blockquote>
 Note that templates cannot be declared
 in a function
</blockquote>

It does not give explanation and/or cross reference to any other chapter in the book or external resource. 

Could someone help in explaining this. Probably it is explained later in the book but not there yet. If explained earlier, I must have missed it.

Example:

<pre><code>int main()
{
 class DummyClass // This compiles ok
 {
 int object;
 };

 template &lt;typename T&gt; // compile error "expected primary-expression before "template""
 class DummyTemplate
 {
 T object;
 };

 return 0;
}
</code></pre>

I do not understand the error message from gcc either. The error message says:

<pre><code>expected primary-expression before "template"
</code></pre>

Why can't templates be declared in a function?

编译器

阅读，它说请注意，模板不能在函数中声明它不会对书中的任何其他章节或外部资源进行解释和/或交叉引用。有人能帮我解释一下吗。这可能会在书的后面解释，但还没有。如果我之前解释过了，我一定是错过了。示例：int main(){  class DummyClass  //  This compiles ok  {    int ...

问为什么不能在函数中声明模板？
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