what is the wrong with this example in the c++ book of Bjarne Stroustrup

Question

I tried the example in bjarne stroustrup's c++ programming language 4th edition:

#include <iostream>
#include <string>
#include <vector>
#include <list>

using namespace std;

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

template<typename C, typename V>
vector<Iterator<C>> find_all(C& c, V v)
{
    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";
    // p is a str ing::iterator
    list<double> ld {1.1, 2.2, 3.3, 1.1};
    for (auto p : find_all(ld,1.1))
        if (*p!=1.1)
            cerr << "list bug!\n";
    vector<string> vs { "red", "blue", "green", "green", "orange", "green" };
    for (auto p : find_all(vs,"green"))
        if (*p!="green")
            cerr << "vector bug!\n";
    for (auto p : find_all(vs,"green"))
        *p = "ver t";
}

int main()
{
    test();
    return 0;
}

§4.5.1

I compile the code with: g++ test.cpp -o test -g -std=c++11 on ubuntu, but I got the errors below:

iterator_of_iterator_version_2.cpp:9:15: error: expected ‘=’ before ‘<’ token
 using Iterator<T> = typename T::iterator;
               ^
iterator_of_iterator_version_2.cpp:9:15: error: expected type-specifier before ‘<’ token
iterator_of_iterator_version_2.cpp:12:8: error: ‘Iterator’ was not declared in this scope
 vector<Iterator<C>> find_all(C& c, V v)
        ^
iterator_of_iterator_version_2.cpp:12:17: error: template argument 1 is invalid
 vector<Iterator<C>> find_all(C& c, V v)
                 ^
iterator_of_iterator_version_2.cpp:12:17: error: template argument 2 is invalid
iterator_of_iterator_version_2.cpp:12:18: error: expected unqualified-id before ‘>’ token
 vector<Iterator<C>> find_all(C& c, V v)
                  ^
iterator_of_iterator_version_2.cpp: In function ‘void test()’:
iterator_of_iterator_version_2.cpp:24:30: error: ‘find_all’ was not declared in this scope
  for (auto p : find_all(m,'a'))
                              ^
iterator_of_iterator_version_2.cpp:29:31: error: ‘find_all’ was not declared in this scope
  for (auto p : find_all(ld,1.1))
                               ^
iterator_of_iterator_version_2.cpp:33:35: error: ‘find_all’ was not declared in this scope
  for (auto p : find_all(vs,"green"))
                                   ^
iterator_of_iterator_version_2.cpp:36:35: error: ‘find_all’ was not declared in this scope
  for (auto p : find_all(vs,"green"))
                                   ^

So what is the problem?

It seems that no syntax error can be found in this code because I just did a copy-paste of the example of the book.

Answer 1

Change the beginning from

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

to

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

Works on my ubuntu 16.04 with probably same compiler setup

Why is it that way? I am not 100percent confident about that, someone else might verify it please.

Writing

using Iterator<T>

is invalid, because it doesnt make sense here. We want Iterator to be a templated typedef that will ask its parameter-type for its generic iterator type. Iterator<T> would specialize the template. E.g. we know it better for a specific type:

template<>
using Iterator<MyClass> = MyClassIterator;

At least that works with normal template classes, i think its the same with using as well.

Answer 2

Change this

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

to this

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