输出流作为类成员

Question

我有一个c ++库,它提供了一个复杂逻辑的对象.在数据处理期间,此对象向std :: cout输出大量内容(现在这是硬编码的).我希望处理输出不是转到标准输出而是转到custm小部件(一些文本显示).我尝试创建一个std::ostream类成员,使用参数设置它(std :: cout用于控制台应用程序,其他一些ostream在GUI应用程序中处理).但是编译器会抛出以下错误:

[ 14%] Building CXX object src/core/CMakeFiles/PietCore.dir/pvirtualmachine.cpp.o
/usr/include/c++/4.6/ostream: In constructor ‘PVirtualMachine::PVirtualMachine(QString)’:                                                                        
/usr/include/c++/4.6/ostream:363:7: error: ‘std::basic_ostream::basic_ostream() [with _CharT = char, _Traits = std::char_traits]’ is protected
/home/tomasz/Development/C++/piet/src/core/pvirtualmachine.cpp:33:50: error: within this context
In file included from /usr/include/c++/4.6/ios:45:0,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iterator:64,
                 from /usr/include/qt4/QtCore/qlist.h:50,
                 from /usr/include/qt4/QtCore/qvector.h:48,
                 from /usr/include/qt4/QtGui/qpolygon.h:45,
                 from /usr/include/qt4/QtGui/qmatrix.h:45,
                 from /usr/include/qt4/QtGui/qtransform.h:44,
                 from /usr/include/qt4/QtGui/qimage.h:45,
                 from /usr/include/qt4/QtGui/QImage:1,
                 from /home/tomasz/Development/C++/piet/src/core/pcodepointer.h:17,
                 from /home/tomasz/Development/C++/piet/src/core/pblockmanager.h:9,
                 from /home/tomasz/Development/C++/piet/src/core/pvirtualmachine.h:10,
                 from /home/tomasz/Development/C++/piet/src/core/pvirtualmachine.cpp:4:
/usr/include/c++/4.6/bits/ios_base.h: In member function ‘std::basic_ios& std::basic_ios::operator=(const std::basic_ios&)’:
/usr/include/c++/4.6/bits/ios_base.h:791:5: error: ‘std::ios_base& std::ios_base::operator=(const std::ios_base&)’ is private
/usr/include/c++/4.6/bits/basic_ios.h:64:11: error: within this context
In file included from /usr/include/c++/4.6/iterator:64:0,
                 from /usr/include/qt4/QtCore/qlist.h:50,
                 from /usr/include/qt4/QtCore/qvector.h:48,
                 from /usr/include/qt4/QtGui/qpolygon.h:45,
                 from /usr/include/qt4/QtGui/qmatrix.h:45,
                 from /usr/include/qt4/QtGui/qtransform.h:44,
                 from /usr/include/qt4/QtGui/qimage.h:45,
                 from /usr/include/qt4/QtGui/QImage:1,
                 from /home/tomasz/Development/C++/piet/src/core/pcodepointer.h:17,
                 from /home/tomasz/Development/C++/piet/src/core/pblockmanager.h:9,
                 from /home/tomasz/Development/C++/piet/src/core/pvirtualmachine.h:10,
                 from /home/tomasz/Development/C++/piet/src/core/pvirtualmachine.cpp:4:
/usr/include/c++/4.6/ostream: In member function ‘std::basic_ostream& std::basic_ostream::operator=(const std::basic_ostream&)’:
/usr/include/c++/4.6/ostream:57:11: note: synthesized method ‘std::basic_ios& std::basic_ios::operator=(const std::basic_ios&)’ first required here 
/home/tomasz/Development/C++/piet/src/core/pvirtualmachine.cpp: In member function ‘void PVirtualMachine::setOutput(std::ostream)’:
/home/tomasz/Development/C++/piet/src/core/pvirtualmachine.cpp:216:11: note: synthesized method ‘std::basic_ostream& std::basic_ostream::operator=(const std::basic_ostream&)’ first required here

如果有人指出我出了什么问题,我会很高兴,因为我不知道......

我的代码看起来像这样:

• .h文件

class PVirtualMachine {
  private:
    std::ostream output;
    [...]
  public:
    void setOutput(std::ostream);
    [...]
};

• .cpp文件

void PVirtualMachine::setOutput(std::ostream os)
{
  output = os;
}

Answer 1

你有两个选择:

• 使用引用,或
• 使用指针

您不能使用普通实例,因为它ostream是不可复制的.

使用引用(直接引用已经实例化的ostream)

class PVirtualMachine {
  private:
    std::ostream & output;
    [...]
  public:
    PVirtualMachine(std::ostream &);  // Reference must be initialized on construction.
    [...]
};

好处:

• 没有指针语法.
• 没有内存泄漏的风险.

缺点:

• 如果原始实例超出范围,则引用可能变为无效.
• 该std::ostream班必须在初始化列表输出参考构造,否则将无法编译.
• 初始化后无法更改引用.

使用指针(引用堆上的对象)

class PVirtualMachine {
  private:
    std::ostream * output;
    [...]
  public:
    void setOutput(std::ostream *);
    [...]
};

好处:

• 可以实例化为空指针.
• 可以轻松传递.
• 可以在PVirtualMachine实例的内部或外部创建.

缺点:

• 指针语法.
• 必须手动管理分配/删除(除非您使用智能指针).
• 访问ostream时必须检查空引用.