如何使用模板在C++中使用`using`(制作参数化别名)创建别名？

Question

我正在阅读Bjarne Stroustrup的"The C++ Programming Language"第4版.在本书的第一部分中,我发现了using如下所示的用法:

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

*有关完整程序和错误消息,请参阅[**]*

这正是我在页面105中找到的.当我把它变成一个完整的程序并试图编译它时,g++给了我这个错误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

我在这段代码中找不到任何问题,(我是C++的新手,我用我的小知识找不到问题)(更混乱的是我在Bjarne的书上发现了这个)

有人能告诉我为什么那段代码会出错？

注:不过如果我换成Iterator<C>用typename C::iterator(见下文),它工作正常,没有错误!

[**]完整程序和错误消息:

// 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

首先<T>必须省略

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

Answer 2

定义类模板或函数模板时,使用:

template <typename T> struct Foo { };

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

您不使用Foo<T>或bar<T>在定义模板时.

同样,使用模板定义别名时,您需要使用:

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