C多态与引用
我一直在用C编写一些操作文件的类.当我开始时,我让所有构造函数接受一个被解释为文件名的std :: string参数,因为这很快捷.
任何类使用的唯一函数std :: fstream是.open().我认为既然std :: ofstream和std :: ostringstream都来自std :: ostream而我只使用std :: ostream的函数,那么让每个构造函数采用std :: ostream或std会好得多: :istream参数适当,而不是将类绑定到文件. 一个伟大的教科书例子,为什么多态是有用的.海湾合作委员会对此非常不满.这是我的代码的简化: #include <iostream> #include <fstream> class Foo { public: Foo(std::istream&); std::istream& input; }; Foo::Foo(std::istream& is) { this->input = is; } int main() { std::ifstream is("test.txt"); Foo foo(is); } 当使用g test.cpp编译时,产生以下各种各样的错误: test.cpp: In constructor ‘Foo::Foo(std::istream&)’: test.cpp:10:1: error: uninitialized reference member ‘Foo::input’ [-fpermissive] In file included from /usr/include/c++/4.7/ios:43:0,from /usr/include/c++/4.7/ostream:40,from /usr/include/c++/4.7/iostream:40,from test.cpp:1: /usr/include/c++/4.7/bits/ios_base.h: In member function ‘std::basic_ios<char>& std::basic_ios<char>::operator=(const std::basic_ios<char>&)’: /usr/include/c++/4.7/bits/ios_base.h:791:5: error: ‘std::ios_base& std::ios_base::operator=(const std::ios_base&)’ is private In file included from /usr/include/c++/4.7/ios:45:0,from test.cpp:1: /usr/include/c++/4.7/bits/basic_ios.h:64:11: error: within this context In file included from /usr/include/c++/4.7/iostream:41:0,from test.cpp:1: /usr/include/c++/4.7/istream: In member function ‘std::basic_istream<char>& std::basic_istream<char>::operator=(const std::basic_istream<char>&)’: /usr/include/c++/4.7/istream:56:11: note: synthesized method ‘std::basic_ios<char>& std::basic_ios<char>::operator=(const std::basic_ios<char>&)’ first required here test.cpp: In constructor ‘Foo::Foo(std::istream&)’: test.cpp:11:16: note: synthesized method ‘std::basic_istream<char>& std::basic_istream<char>::operator=(const std::basic_istream<char>&)’ first required here 我无法筛选所有模板参数,但看起来它不喜欢这个 – > input = is;.每个类都能够作为成员访问流是很重要的,因为各种不同的成员函数都必须能够看到它.我认为GCC可能对指针更好一些,但对我来说,指向类的指针看起来像是动态分配的内存,我只想提出这些怀疑,如果它们是真的.什么是正确的方法? 解决方法
必须在
constructor initialization list中初始化引用类型的成员(以及const类型的成员):
Foo::Foo(std::istream& is) : input(is) // ^^^^^^^^^^^ { } 原因是您需要在使用它之前立即初始化引用.您原始代码中的内容是非法的,原因与此相同: int y = 42; int& x; // ERROR! x = y; 另一方面,初始化列表保证在输入构造函数体之前,引用成员绑定到对象. (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |