为我在 C++ 中创建的对象声明变量

Question

我是一名 Java 人员，我正在尝试用 C++ 为一门课做作业。我在将我创建的类型的变量声明为另一个类的属性时遇到问题。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;
    }

，我收到此错误

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;
            ^^

我相信您的意图是创建一个 HeatingUnit 类型的指针，因为您在构造函数中使用了 new 。 new 用于将自由存储上的内存分配给指针变量。

如果创建指针不是您的意图（我不确定，因为您是从没有指针的 java 迁移的），只需使用：

   HeatingUnit heatingUnit;

但不要使用 `new 在构造函数中分配内存。

Answer 2

在 C++ 中，对象是在没有 new 的情况下创建的，除非您想将它们放在堆上。要将它们放在堆上，您需要使用指针，例如

HeatingUnit* heatingUnit;

但是，您真正想要的是从成员初始值设定项列表中初始化对象：

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

Answer 3

变量声明应该是，

HeatingUnit *heatingUnit;

new HeatingUnit 返回一个指针HeatingUnit* 并保存您需要一个指针。

供您参考，这里有另一种方法（它有其局限性，但您感觉有点像 Java）：

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

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

请注意，我们在构造函数的初始值设定项列表中定义 heatingUnit。您也可以对 tempMaintain 执行此操作。

现在，heatingUnit 可以像 Java 一样与 . 运算符一起使用，而不是 ->。

除此之外，在 C++ 中，只有当您想动态执行时才应该使用 new。在您的具体情况下， HeatingUnit heatUnit; 就足够了，它声明一个自动对象，不需要手动进行任何释放。

Answer 4

如果你想做：

heatingUnit= new HeatingUnit(isOn, initialTemp);

你必须改为

HeatingUnit heatingUnit

为什么

HeatingUnit *heatingUnit

？因为在 Java 中，默认情况下所有对象只能保存对对象的引用，而要在 C++ 中做到这一点，您必须将其声明为指针，而在 C++ 中，对象可以将子对象作为字段（可以不能用 Java 完成）。

此外，虽然在 Java 中，初始化是在构造函数主体中完成的，但在 C++ 中使用初始化列表是一个很好的做法。

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

但是，使用 new 要求您必须使用 delete 自行清理内存，因此以下内容可能更可取：

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