模板中依赖类型的问题

Question

我遇到模板和依赖类型的问题:

namespace Utils
{
    void PrintLine(const string& line, int tabLevel = 0);
    string getTabs(int tabLevel);

    template<class result_t, class Predicate>
    set<result_t> findAll_if(typename set<result_t>::iterator begin, set<result_t>::iterator end, Predicate pred) // warning C4346
    {
        set<result_t> result;
        return findAll_if_rec(begin, end, pred, result);
    }
}

namespace detail
{
    template<class result_t, class Predicate>
    set<result_t> findAll_if_rec(set<result_t>::iterator begin, set<result_t>::iterator end, Predicate pred, set<result_t> result)
    {
        typename set<result_t>::iterator nextResultElem = find_if(begin, end, pred);
        if (nextResultElem == end)
        {
            return result;
        }
        result.add(*nextResultElem);

        return findAll_if_rec(++nextResultElem, end, pred, result);
    }
}

编译器投诉,来自上述位置:

warning C4346: 'std::set<result_t>::iterator' : dependent name is not a type. prefix with 'typename' to indicate a type
error C2061: syntax error : identifier 'iterator'

我究竟做错了什么？

Answer 1

嗯,警告说:

依赖名称不是类型.带有'typename'的前缀表示类型

从属名称(即iteratorin std::set<result_t>::iterator)不是类型.您需要在其前面加上typename以指示类型:

typename std::set<result_t>::iterator

所以,你的声明应该是:

template<class result_t, class Predicate>
set<result_t> findAll_if(typename set<result_t>::iterator begin, typename set<result_t>::iterator end, Predicate pred)
                                                note added typename ^

(并且定义应与声明相符)

Answer 2

您需要在此行中添加其他typename关键字:

set<result_t> findAll_if(typename set<result_t>::iterator begin,类型名set<result_t>::iterator end, Predicate pred) // warning C4346