为我在C++中创建的对象声明变量

Question

我是一个Java人，我正在尝试使用C++为一个类进行任务。我无法声明作为另一个类的属性创建的类型的变量。这是我的方法

private: 
    HeatingUnit heatingUnit; 
    int tempToMaintain; 

    public: 
    BangBangControl(int tempToMaintain, bool isOn, int initialTemp){ 
     heatingUnit= new HeatingUnit(isOn, initialTemp); 
     this -> tempToMaintain = tempToMaintain; 
    } 

我得到这个错误

BangBangControl.cpp: In constructor ‘BangBangControl::BangBangControl(int, bool, int)’: 
BangBangControl.cpp:15: error: no match for ‘operator=’ in ‘((BangBangControl*)this)->BangBangControl::heatingUnit = (((HeatingUnit*)operator new(8u)), (<anonymous>->HeatingUnit::HeatingUnit(((int)isOn), initialTemp), <anonymous>))’ 
HeatingUnit.h:6: note: candidates are: HeatingUnit& HeatingUnit::operator=(const HeatingUnit&) 
BangBangControl.cpp: In member function ‘int BangBangControl::main()’: 
BangBangControl.cpp:37: error: no match for ‘operator<<’ in ‘std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)(& std::cout)), ((const char*)"Temp to maintain is: ")))->std::basic_ostream<_CharT, _Traits>::operator<< [with _CharT = char, _Traits = std::char_traits<char>](bBC. BangBangControl::getTemp()))), ((const char*)" Current temp is: ")) << bBC. BangBangControl::update()’ 
/usr/include/c++/4.2.1/ostream:112: note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>& (*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:121: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>& (*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:131: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:169: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:173: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:177: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/bits/ostream.tcc:92: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:184: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/bits/ostream.tcc:106: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:195: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:204: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:208: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:213: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:217: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:225: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/ostream:229: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>] 
/usr/include/c++/4.2.1/bits/ostream.tcc:120: note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>] 

Answer 1

HeatingUnit heatingUnit; 

应该是：

HeatingUnit *heatingUnit; 
      ^^ 

我相信你的意图是，因为你创建的类型HeatingUnit的指针在构造函数中使用new。 new用于将空闲存储上的内存分配给指针变量。

如果创建一个指针是不是你的意图（我不知道，因为你是从没有指针的Java迁移），只需使用：

HeatingUnit heatingUnit; 

但不要用'新来分配内存中的构造函数。

Answer 2

变量声明应该是，

HeatingUnit *heatingUnit; 

new HeatingUnit返回一个指针HeatingUnit*，并认为你需要一个指针。

为了您的信息，这里是另一种方式（它有其局限性，但你觉得有点像Java）：

private: 
    HeatingUnit &heatingUnit; // <--- declare reference 
    int tempToMaintain; 

    public: 
    BangBangControl(int tempToMaintain, bool isOn, int initialTemp) : 
    heatingUnit(*new HeatingUnit(isOn, initialTemp)) 
    { 
     this -> tempToMaintain = tempToMaintain; 
    } 

需要注意的是，我们在构造函数初始化列表定义heatingUnit。你也可以为tempMaintain做这个。

现在heatingUnit可以与.运算符一起使用，而不是像Java那样的->。

所有这些除了C++之外，只有在动态执行时才应该使用new。在你的具体情况下，HeatingUnit heatingUnit;就足够了，它声明了一个自动对象，并且不需要任何手动解除分配。

Answer 3

在C++对象中创建时没有new，除非你想把它们放在堆上。要把它们放在堆上，你需要使用一个指针，例如

HeatingUnit* heatingUnit; 

但是，你真正想要的是从成员初始化列表初始化对象：

BangBangControl(int tempToMaintain_, bool isOn, int initialTemp): 
    heatingUnit(isOn, initialTemp), 
    tempToMaintain(tempToMaintain_) 
{ 
} 

Answer 4

如果你想做的事：

heatingUnit= new HeatingUnit(isOn, initialTemp); 

你必须改变

HeatingUnit heatingUnit 

到

HeatingUnit *heatingUnit 

为什么？因为在Java中，所有对象只能将引用默认为对象，要在C++中执行该操作，则必须将其声明为指针，而在C++中，对象可以具有子对象作为字段（不能在使用Java）。

另外，在Java中，初始化是在构造函数的主体中完成的，在C++中使用初始化列表是很好的做法。

BangBangControl(int tempToMaintain, bool isOn, int initialTemp) 
: heatingUnit(new HeatingUnit(isOn, initialTemp)), 
    tempToMaintain(tempToMaintain) 
{ } 

但是，使用new要求，你必须清理自己使用delete所以下面将可能是最好的记忆：

BangBangControl(int tempToMaintain, bool isOn, int initialTemp) 
: heatingUnit(isOn, initialTemp), 
    tempToMaintain(tempToMaintain) 
{ }