防止const成员函数更改成员数组 [英] Preventing a const member function from changing a member array

问题描述

显然，仍然允许使用const成员函数更改类成员指向的数据。这是我的意思的示例：

Apparently, a const member function is still allowed to change data that the class member are pointing to. Here's an example of what I mean:

class MyClass
{
public:
  MyClass();
  int getSomething() const;
private:
  int* data;
};

// ... data = new int[10];, or whatever

int MyClass::getSomething() const
{
  data[4] = 3; // this is allowed, even those the function is const
  return data[4];
}

我希望这样做是不允许的。我应该如何定义数据，以便不允许 getSomething（）const进行更改？ （但是允许非const函数更改它。）对此是否存在某种最佳实践？也许是std :: vector？

I'd prefer if this was not allowed. How should I define "data" so that "getSomething() const" isn't allowed to change it? (but so that non-const functions are allowed to change it.) Is there some kind of "best practice" for this? Perhaps std::vector?

推荐答案

在 const 成员函数中， 数据的类型从 int * 更改为 int * const ：

In a const member function, the type of data changes from int* to int *const:

int * const data;

这意味着指针是 const 在const成员函数中，而不是指针指向的数据本身。因此，您无法执行以下操作：

which means, it's the pointer which is const in the const member function, not the data itself the pointer points to. So you cannot do the following:

data = new int[100]; //error 

，因为它试图更改指针本身，即const，因此是不允许的，但是以下允许：

as it's attempting to change the pointer itself which is const, hence disallowed, but the following is allowed:

data[0] = 100; //ok

因为更改内容不会更改指针。 data 指向相同的内存位置。

Because changing the content doesn't change the pointer. data points to same memory location.

如果使用 std :: vector< int> ; ，那么您可以实现自己想要的。实际上，矢量解决了这个问题，并因此解决了内存管​​理问题：

If you use std::vector<int>, then you can achieve what you want. In fact, vector solves this problem, along with the memory management issues, therefor use it:

class MyClass
{
public:
  MyClass();
  int getSomething() const;
private:
  std::vector<int> data;
};

MyClass::MyClass() : data(10) {}  //vector of size 10

int MyClass::getSomething() const
{
  data[4] = 3; // compilation error - this is what you wanted.
  return data[4];
}

尽可能避免使用非RAII设计。 RAII是解决内存管理问题的出色解决方案。在这里，您可以实现自己想要的。阅读此内容：

Avoid non-RAII design as much as you can. RAII is superior solution to memory management issues. Here, with it, you achieve what you want. Read this:

• 资源获取是初始化（RAII）

• Resource Acquisition Is Initialization (RAII)

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