从 const 成员函数返回非常量引用

2021-12-13 00:00:00 reference pointers constants c++

为什么返回对指向的成员变量的引用有效，而另一个无效?我知道 const 成员函数应该只返回 const 引用，但为什么对于指针来说这似乎不是真的?

Why does returning the reference to a pointed-to member variable work, but not the other? I know that a const member function should only return const references, but why does that not seem true for pointers?

class MyClass
{
  private:
    int * a;
    int b;
  public:
    MyClass() { a = new int; }
    ~MyClass() { delete a; }

    int & geta(void) const { return *a; } // good?
    int & getb(void) const { return b; }  // obviously bad
};

int main(void)
{
  MyClass m;

  m.geta() = 5;  //works????
  m.getb() = 7;  //doesn't compile

  return 0;
}

推荐答案

int & geta(void) const { return *a; } // good?
int & getb(void) const { return b; }  // obviously bad

在一个 const 函数中，每个数据成员都变成了 const 这样它就不能被修改.int 变为 const int，int * 变为 int * const，依此类推.

In a const-function, every data member becomes const in such way that it cannot be modified. int becomes const int, int * becomes int * const, and so on.

因为在你的第一个函数中 a 的 type 变成了 int * const，而不是 const int *>，因此您可以更改数据(可修改):

Since the type of a in your first function becomes int * const, as opposed to const int *, so you can change the data (which is modifiable):

  m.geta() = 5;  //works, as the data is modifiable

区别:const int* 和 int * const.

  • const int* 表示指针是非常量，但指针指向的数据是const.
  • int * const 表示指针是const，但指针指向的数据是非常量.
  • const int* means the pointer is non-const, but the data the pointer points to is const.
  • int * const means the pointer is const, but the data the pointer points to is non-const.

你的第二个函数试图返回 const int &，因为 b 的 type 变成了 const int.但是您已经在代码中将实际返回类型提到为 int &，因此该函数甚至不会 编译(请参阅 this)，无论您在 main() 中做什么，因为返回的 type 不匹配.这是修复:

Your second function tries to return const int &, since the type of b become const int. But you've mentioned the actual return type in your code as int &, so this function would not even compile (see this), irrespective of what you do in main(), because the return type doesn't match. Here is the fix:

 const int & getb(void) const { return b; }

现在它编译正常！.

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