这个例子在Bjarne Stroustrup的c ++书中有什么问题

Question

我在bjarne stroustrup的c ++编程语言第4版中尝试了这个例子:

#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

我在ubuntu上用以下代码编译代码:g ++ test.cpp -o test -g -std = c ++ 11,但是我得到了以下错误:

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"))
                                   ^

那么问题是什么？

似乎在此代码中找不到语法错误,因为我只是对该书的示例进行了复制粘贴.

Answer 1

改变开头

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

至

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

适用于我的ubuntu 16.04,可能与编译器设置相同

为什么会那样？我不是100%对此有信心,其他人可能会对此进行验证.

写作

using Iterator<T>

是无效的,因为它在这里没有意义.我们希望Iterator是一个模板化的typedef,它会将其参数类型作为其泛型迭代器类型. Iterator<T>会专门化模板.例如,我们知道特定类型更好:

template<>
using Iterator<MyClass> = MyClassIterator;

至少这适用于普通的模板类,我认为它与使用相同.