如何使用模板在 C++ 中使用 `using`（制作参数化别名）制作别名？

Question

我目前正在阅读 Bjarne Stroustrup 的 "The C++ Programming Language" 第 4 版。在本书的第一部分，我发现 using 的用法如下所示：

// Error is in these 2 lines
template<typename T>
using Iterator<T> = typename T::iterator;

*see [**] for complete program and error message*

这正是我在第 105 页中找到的内容。当我将其转换为一个完整的程序并尝试编译它时，g++ 给了我这个错误信息：

> g++ -std=c++14 -o fnd find_all.cpp
find_all.cpp:13:15: error: expected '=' before '<' token
 using Iterator<T> = typename T::iterator;
           ^
find_all.cpp:13:15: error: expected type-specifier before '<' token

我在这段代码中找不到任何问题，（我是C++的新手，我的知识很少）（更令人困惑的是我在 Bjarne 的书上找到的）

Could someone tell me why does that code makes an error?

NOTE: However If I replaced Iterator<C> with typename C::iterator (see below), It works fine, there is no error!

[**]完整的程序和错误信息：

// Error is in these 2 lines
template<typename T>
using Iterator<T> = typename T::iterator;
// -------------------------------------------

// For the completeness I'll include my complete program here
template<typename C, typename V>
vector<Iterator<C>> find_all(C& c, V v) // find all occurrences of v in c
{
    vector<Iterator<C>> res;
    for (auto p = c.begin(); p!=c.end(); ++p)
        if (∗p==v)
            res.push_back(p);
    return res;
}

void test()
{
    string m {"Mary had a little lamb"};
    for (auto p : find_all(m, 'a'))
        if (*p == 'a')
            cerr << "string bug!\n";

    list<double> ld { 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 1.1, 1.1 };
    for (auto p : find_all(ld, 1.1))
        if (*p == 1.1)
            cerr << "list bug!\n";

    vector<string> strv { "blue", "yellow", "red", "white", "orange", "blue" };
    for (auto p : find_all(strv, "blue"))
        if (*p == "blue")
            cerr << "string vector bug!\n";

}

int main(void) 
{
    test();

    return 0;
}

错误信息：

> g++ -std=c++14 -o fnd find_all.cpp
find_all.cpp:13:15: error: expected '=' before '<' token
 using Iterator<T> = typename T::iterator;
           ^
find_all.cpp:13:15: error: expected type-specifier before '<' token
find_all.cpp:16:8: error: 'Iterator' was not declared in this scope
 vector<Iterator<C>> find_all(C& c, V v)
    ^
find_all.cpp:16:17: error: template argument 1 is invalid
 vector<Iterator<C>> find_all(C& c, V v)
             ^
find_all.cpp:16:17: error: template argument 2 is invalid
find_all.cpp:16:18: error: expected unqualified-id before '>' token
 vector<Iterator<C>> find_all(C& c, V v)
              ^
find_all.cpp: In function 'void test()':
find_all.cpp:30:31: error: 'find_all' was not declared in this scope
  for (auto p : find_all(m, 'a'))
                           ^
find_all.cpp:35:32: error: 'find_all' was not declared in this scope
  for (auto p : find_all(ld, 1.1))
                            ^
find_all.cpp:40:37: error: 'find_all' was not declared in this scope
  for (auto p : find_all(strv, "blue"))

Answer 1

第一个<T>必须省略

template<typename T>
using Iterator = typename T::iterator;

Answer 2

定义 class 模板或函数模板时，您使用：

template <typename T> struct Foo { };

template <typename T> T bar() { return T{}; }

您在定义模板时不使用 Foo<T> 或 bar<T>。

同样，使用模板定义别名时，需要使用：

template <typename T>
using Iterator = typename T::iterator;
             ^^ Don't include <T>

如何使用模板在 C++ 中使用 `using`（制作参数化别名）制作别名？

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