• <ins id="pjuwb"></ins>
    <blockquote id="pjuwb"><pre id="pjuwb"></pre></blockquote>
    <noscript id="pjuwb"></noscript>
          <sup id="pjuwb"><pre id="pjuwb"></pre></sup>
            <dd id="pjuwb"></dd>
            <abbr id="pjuwb"></abbr>
            posts - 9, comments - 0, trackbacks - 0, articles - 0
              C++博客 :: 首頁 :: 新隨筆 :: 聯系 :: 聚合  :: 管理

            類模板中的友元函數

            Posted on 2013-01-21 10:48 魏尚堂 閱讀(592) 評論(0)  編輯 收藏 引用
            template <typename T> class dataList
            {
               public:
                  friend ostream& operator<<(ostream& outStream, const dataList <T> &outList);
            }
            template <typename T> ostream& operator<<(ostream& outStream, const dataList <T> &outList)
            {
               //....
               return outStream;
            }
            int main(int argc, char* argv[])
            {
               dataList <int> testList;
               cout << testList;
            }
            這個程序員是鏈接不過,
            錯誤信息:
             warning: friend declaration âstd::ostream& operator<<(std::ostream&, const dataList<T>&)â declares a non-template function
             note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)
            /tmp/cc9DSuka.o: In function `main':
            undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, dataList<int> const&)'
            collect2: ld returned 1 exit status
            錯誤原因解釋

            The problem is that the compiler is not trying to use the templated operator<< you provided, but rather a non-templated version.

            When you declare a friend inside a class you are injecting the declaration of that function in the enclosing scope. The following code has the effect of declaring (and not defining) a free function that takes a non_template_test argument by constant reference:

            class non_template_test { friend void f( non_template_test const & ); }; // declares here: // void f( non_template_test const & );

            The same happens with template classes, even if in this case it is a little less intuitive. When you declare (and not define) a friend function within the template class body, you are declaring a free function with that exact arguments. Note that you are declaring a function, not a template function:

            template<typename T> class template_test { friend void f( template_test<T> const & t ); }; // for each instantiating type T (int, double...) declares: // void f( template_test<int> const & ); // void f( template_test<double> const & );  int main() {     template_test<int> t1;     template_test<double> t2; }

            Those free functions are declared but not defined. The tricky part here is that those free functions are not a template, but regular free functions being declared. When you add the template function into the mix you get:

            template<typename T> class template_test { friend void f( template_test<T> const & ); }; // when instantiated with int, implicitly declares: // void f( template_test<int> const & );  template <typename T> void f( template_test<T> const & x ) {} // 1  int main() {    template_test<int> t1;    f( t1 ); }

            When the compiler hits the main function it instantiates the template template_test with type intand that declares the free function void f( template_test<int> const & ) that is not templated. When it finds the call f( t1 ) there are two f symbols that match: the non-template f( template_test<int> const & ) declared (and not defined) when template_test was instantiated and the templated version that is both declared and defined at 1. The non-templated version takes precedence and the compiler matches it.

            When the linker tries to resolve the non-templated version of f it cannot find the symbol and it thus fails.

            What can we do? There are two different solutions. In the first case we make the compiler provide non-templated functions for each instantiating type. In the second case we declare the templated version as a friend. They are subtly different, but in most cases equivalent.

            Having the compiler generate the non-templated functions for us:

            template <typename T> class test  { friend void f( test<T> const & ) {} }; // implicitly

            This has the effect of creating as many non-templated free functions as needed. When the compiler finds the friend declaration within the template test it not only finds the declaration but also the implementation and adds both to the enclosing scope.

            Making the templated version a friend

            To make the template a friend we must have it already declared and tell the compiler that the friend we want is actually a template and not a non-templated free function:

            template <typename T> class test; // forward declare the template class template <typename T> void f( test<T> const& ); // forward declare the template template <typename T> class test { friend void f<>( test<T> const& ); // declare f<T>( test<T> const &) a friend }; template <typename T> void f( test<T> const & ) {}

            In this case, prior to declaring f as a template we must forward declare the template. To declare the ftemplate we must first forward declare the test template. The friend declaration is modified to include the angle brackets that identify that the element we are making a friend is actually a template and not a free function.
            引用自 http://stackoverflow.com/questions/1810753/overloading-operator-for-a-templated-class
            從上面我可以學到一點:
            1, 編譯器匹配方法時非模板函數優先模板函數
            2, 友元函數模板必須提前聲明

            91精品久久久久久无码| 狠狠精品久久久无码中文字幕| 国产成人精品久久一区二区三区| 日韩精品无码久久久久久| 91精品国产高清91久久久久久| 久久综合中文字幕| 久久人妻无码中文字幕| 久久99国产精品久久| 久久亚洲精品无码VA大香大香| 无码精品久久久天天影视| 久久久精品久久久久久| 中文成人久久久久影院免费观看| 久久久久久亚洲精品成人 | 久久久久人妻精品一区二区三区| 久久国产精品久久久| 久久久国产打桩机| 精品乱码久久久久久夜夜嗨| 日产精品久久久久久久| 亚洲精品无码久久毛片 | 国产精品热久久毛片| 久久精品青青草原伊人| 久久久久一本毛久久久| 99久久这里只有精品| 亚洲va久久久噜噜噜久久| 久久人人爽人人爽人人片AV东京热| 久久国产乱子伦精品免费强| 中文字幕日本人妻久久久免费 | 国产精品美女久久久久久2018| 午夜视频久久久久一区| 国产精品gz久久久| 18岁日韩内射颜射午夜久久成人| 99精品久久久久久久婷婷| 久久久久亚洲AV成人网人人网站 | 久久超乳爆乳中文字幕| 少妇久久久久久久久久| 99久久国产宗和精品1上映| 少妇熟女久久综合网色欲| 免费精品国产日韩热久久| 热久久视久久精品18| 久久大香萑太香蕉av| 成人久久免费网站|