问题描述
我（模糊地）知道，如果不使用，则不会实例化模板。例如，以下代码将编译良好，即使 T :: type 在 T = int 。
I (vaguely) know that a template is not instantiated if it is not used. For example, the following code will compile fine even though T::type doesn't make sense when T = int.
template<typename T>
struct A
{
void f() { using type = typename T::type; }
};
A<int> a; //ok
它编译，因为 f（）不是 ，因此不会实例化—因此 T :: type 的有效性保持未选中状态。如果其他成员函数 g（）调用 f（） 。
It compiles because f() is not used, so it is not instantiated — thus the validity ofT::type remains unchecked. It doesn't matter if some other member function g() calls f().
template<typename T>
struct A
{
void f() { using type = typename T::type; }
void g() { f(); } //Is f() still unused?
};
A<int> a; //ok
这也是。但在这里，我意识到在我理解的定义使用的模糊性。我问：
This also compile fines. But here I realize the vagueness in my understanding of the definition of "use". I ask:
- 是
f（）怎么样？
我可以清楚地看到它在 g（）中使用。但是我认为由于 g（）不使用， f（）视图。这似乎够合理了。
I can clearly see it being used inside g(). But then I thought since g() is not used, f() is not used either, from instantiation point of view. That seems reasonable enough. so far.
但是如果我把 virtual 关键字添加到 g c $ c>，它不编译：
However if I add virtual keyword to g(), it doesn't compile:
template<typename T>
struct A
{
void f() { using type = typename T::type; }
virtual void g() { f(); } //Now f() is used? How exactly?
};
A<int> a; //error
结果是，因为现在它尝试实例化 f（）。我不明白这个行为。
It results in compilation error because now it attempts to instantiate f(). I don't understand this behavior.
有人可以解释一下吗？特别是 virtual 关键字对类模板成员使用的定义的影响。
Could anybody explain this? Especially the impact of virtual keyword on the definition of "use" of member of class template.
推荐答案
快速浏览一下3.2 [basic.def.odr]，得到：
A quick look at 3.2 [basic.def.odr] yields:
我也在14.7.1 [temp.inst]
And I also found at 14.7.1 [temp.inst]:
所以...我会说一个 virtual 方法很可能会被实例化。
So... I would say it is likely that a virtual method will always be instantiated.
在实用术语中，我期望编译器在实例化类时实例化模板类的虚拟表;并因此立即实例化该类的所有 virtual 成员函数（因此它可以引用来自虚拟表的那些）。
In pragmatic terms, I would expect a compiler to instantiate the virtual table of a template class when it instantiates the class; and thus immediately instantiate all virtual member functions of this class (so it can references those from the virtual table).
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