c_cpp 常量的用法

Posted 2021-05-14

#include <iostream>

class A
{
public:
  A(int aValue)
    : mValue(aValue)
  {
  }

  ~A()
  {
  }

  void setValue(int aValue)
  {
    mValue = aValue;
  }

  int PlusOne()
  {
    mValue = mValue + 1;
    return mValue;
  }

  // int PlusOne2() const
  // {
  //   mValue = mValue + 1;
  //   return mValue;
  // }
  //
  // Cause error if we uncomment the PlusOne2() above:
  //
  // test.cpp:28:12: error: cannot assign to non-static data member within const member function
  //       'PlusOne2'
  //     mValue = mValue + 1;
  //     ~~~~~~ ^
  // test.cpp:26:7: note: member function 'A::PlusOne2' is declared const here
  //   int PlusOne2() const
  //   ~~~~^~~~~~~~~~~~~~~~
  // 1 error generated.
  //
  // The const added after a member function can prevent the member variables
  // from being changed. The function ends with a const has only
  // "read" privilege. It can only "read" the member variables.
  // It can NOT "write" any member variable. That's why it cause an error.
  //
  // By this property, we could add "const" at the end to all the
  // read-only functions like below.
  int getValue() const
  {
    // mValue = 10; // Cause a compiler error if we try to change mValue here!
    return mValue;
  }

  // On the other hand, if we put const in the return type of functions:
  // const int getValue(), that means we return a int value that cannot be changed.

private:
  int mValue;
};

int main()
{
  A a(3);
  std::cout << a.getValue() << std::endl;
  std::cout << a.PlusOne() << std::endl;
  // std::cout << a.PlusOne2() << std::endl;
  std::cout << a.getValue() << std::endl;
  return 0;
}

// $ g++ class-array.cpp --std=c++11
#include <iostream>

class A
{
public:
  A() {}
  ~A() {}
  void setValue(int v) { value = v; }
  int getValue() { return value; }
private:
  int value;
};

class B
{
public:
  B(int v) 
    : value(v)
  {}
  ~B() {}
  void setValue(int v) { value = v; }
  int getValue() { return value; }
private:
  int value;
};


int main()
{
  A array[3];
  array[0].setValue(10);
  array[1].setValue(12);
  array[2].setValue(16);
  std::cout << array[0].getValue() << std::endl;
  std::cout << array[1].getValue() << std::endl;
  std::cout << array[2].getValue() << std::endl;

  // B list[2];
  // class-array.cpp:38:5: error: no matching constructor for initialization of
  //       'B [2]'
  //   B list[2];
  //     ^
  // class-array.cpp:17:3: note: candidate constructor not viable: requires single
  //       argument 'v', but no arguments were provided
  //   B(int v) 
  //   ^
  // class-array.cpp:14:7: note: candidate constructor
  //       (the implicit copy constructor) not viable: requires 1 argument, but 0
  //       were provided
  // class B
  //       ^
  // 1 error generated.
  B list[2] { {3}, {6} }; // C++ 11 style.
  std::cout << list[0].getValue() << std::endl;
  std::cout << list[1].getValue() << std::endl;
  return 0;
}