使用另一个模板类的嵌套名称说明符专门化一个模板类 [英] specialize a template class using a nested name specifier of another template class

问题描述

我想使用另一个模板类的嵌套名称说明符专门化一个模板类.但是编译器抱怨它无法推导出这段代码.我该怎么办?

template struct convert{//这是外部库中的一个类//这个库的指南告诉我要专攻//将某些功能转换为我自己的类.void foo(T&t) {/* 做某事 */}};模板 结构体{struct A_sub{//我的类};};模板 struct convert::A_sub>{//错误，编译器无法推断};

错误:类模板部分特化包含无法推导的模板参数；这个部分特化永远不会被使用[-Wunusable-partial-specialization]

结构转换::A_sub> {

main.cpp:65:19: 注意:不可推导的模板参数 'T'

模板

产生了 1 个错误.

解决方案

您需要 typename 关键字:

struct convert::A_sub>

但它不会帮助你，因为语言阻止了你想做的事情(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf temp.deduct.type):<块引用>

非推导的上下文是:

—(5.1) 使用限定 ID 指定的类型的嵌套名称说明符

想象一下这样的情况:

template 结构体{使用 C = int;};模板 <typename W>结构 Q;模板struct Q::C>{ ... };...Qq;//应该推导出 A 的哪个参数 T 以及编译器应该如何猜测?

I want to specialize a template class using a nested name specifier of another template class. But the compiler complains that it can't deduce this code. What should I do?

template <typename T>
struct convert{ // this is a class in an extern library
                // the guide of this library tells me to specialize 
                // convert to my own class for some features.
    void foo(T&t) {/* do something */}
};

template <typename T>
struct A{
    struct A_sub{ // my class

    };
};

template <typename T>
struct convert<A<T>::A_sub> { // error, compiler can't deduce

};

error: class template partial specialization contains a template parameter that cannot be deduced; this partial specialization will never be used [-Wunusable-partial-specialization]

struct convert::A_sub> {

main.cpp:65:19: note: non-deducible template parameter 'T'

template

1 error generated.

解决方案

You need typename keyword:

struct convert<typename A<T>::A_sub>

But it won't help you, because language prevents what you want to do (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf temp.deduct.type):

The non-deduced contexts are:

—(5.1) The nested-name-specifier of a type that was specified using a qualified-id

Imagine situation like this:

template <typename T> struct A {
    using C = int;
};
template <typename W> struct Q;
template <typename W> struct Q<typename A<T>::C> { ... };
...
Q<int> q; // which argument T for A should be deduced and how compiler is supposed to guess?

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