如何使用模板在C ++中使用`using`（制作参数化别名）创建别名？

Question

I'm currently reading Bjarne Stroustrup's "The C++ Programming Language" 4th Edition. In the first parts of the book, I found an usage of using looks like following:

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

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

This is exactly what I found in the page 105. When I turned this into a complete program and tried to compile it, g++ gave me this error masseage:

> 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

I can't find any problem in this code, ( I'm new to C++, I can't find the problem with my little knowledge )( More confusingly I found this on Bjarne's book )

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!

[**]Complete Program and Error Message:

// 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;
}

ERROR MESSAGE:

> 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

First <T> must be omitted

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

Answer 2

When you define a class template or function template, you use:

template <typename T> struct Foo { };

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

You don't use Foo<T> or bar<T> when defining the templates.

Similarly, when using templates to define an alias, you need to use:

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