何时需要实例化功能模板定义? [英] When is required instantiation of function template definition?

问题描述

在我阅读完此问题的答案后，我仍然在提问.答案讨论的是[temp.point]中定义的实例化点的位置，但是如果需要实例化，则不会进行评估.

After I read the answer for this question, I am still asking question. The answer talks about location of point of instantiation as defined in [temp.point] but it is not evaluated if the instantiations are required.

[temp.inst] 指定何时需要函数模板专门化实例化，一般规则是:

[temp.inst] specifies when a function template specialization instantiation is required, the general rule is:

实现不得隐式实例化函数模板，变量模板，成员模板，非虚拟成员函数，成员类等，除非需要此类实例化.

此外，在本部分的标准中，声明的实例化和定义的实例化是分开考虑的.

Moreover in some paragraph of this section of the standard, instantiation of declaration and instantiation of definition are considered separately.

让我们考虑以下代码:

static void Yeap0(int);

template <class T>
void Yeap(T a){
    Yop(a);
}

template <class T>
auto Yeap2(T a){
    Yop(a);
    return 0;
}
namespace x{
    struct y{};
}

int main() {
    Yeap0(0);//ok no definition required here
    x::y ax{};
    Yeap(ax); //the declaration is sufficient, 
    Yeap2(ax); //need to deduce auto => definition required
               //compilation error
    return 0;
}

namespace x{
    void Yop(y){}
}

static void Yeap0(int){}

Gcc，Clang和MSVC仅对Yeap2(ax)产生错误，抱怨说Yop在Yeap2实例化时未定义.

Gcc, Clang and MSVC produce only an error for Yeap2(ax), complaining that Yop is not defined at the point of instantiation of Yeap2.

但是不会为Yeap(ax)生成此错误.从基本的角度来看，这似乎是合乎逻辑的，对于无模板功能Yeap0，仅需要声明.

But this error is not generated for Yeap(ax). From basic consideration this seems logical, only the declaration is needed, as for the none template function Yeap0.

但是 [temp.inst]/4 的演讲让我感到困惑.可以理解，Yeap的实例化也是必需的.但是似乎编译器走了一条更聪明的路.

But the lecture of [temp.inst]/4 let me perplex. It could be understood that the instantiation of Yeap is also required. But it seems that compilers have taken a more clever path.

编译器的行为是扩展吗?

Is the compilers behavior an extension?

注意:我将不接受无需诊断"答案.这将是对情报的侮辱:是否可以相信3个编译器已经特别注意为Yeap(ax)这样的情况关闭诊断，而对于Yeap2(ax)不是这种情况?

Note: I will not accept the "no diagnostic required" answer. This would be an insult to intelligence: Could one believe that 3 compilers have taken special care to shut down diagnostics for case like Yeap(ax) but not for Yeap2(ax)?

推荐答案

编译器抱怨的真正原因是该函数返回auto.

The compiler complaining really comes from the fact that the function returns auto.

在auto情况下，您确实击中

除非需要此类实例化.

必须来自要求 dcl.spec.auto >.

要验证评估，您可以在代码Yeap2中替换为:

To verify that assessment you can replace in your code Yeap2 by:

template <class T>
auto Yeap2(T a){
    Yeap(a);
    return 0;
}

并且没有编译错误.

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