<分区>
我在 C++ 中有一个数据结构类，它带有某个对象(可能很大)的访问器，并且我有使用此访问器的 const 和非 const 方法，因此我需要重载它。我正在寻找对下面代码的批评 - 也许有一种方法可以完成同样的事情但更干净？
在我的理解中，有两种方法可以在不复制访问器代码的情况下实现这一点，在下面的例子中，get() 方法。我不确定这两种方法中的任何一种是否存在严重问题，我需要一些指导。
我喜欢方法 A，因为:
- 只有一个const_cast
- 方法 get() 的常量版本返回一个拷贝
- 非常量方法直接获取非常量引用
我不喜欢方法 A，因为:
- 非 const 方法 get() 仅按约定为 const，(未经编译器检查)
- 更难获得常量引用，但并非不可能
我喜欢方法 B，因为:
- const 方法 get() 的 const-ness 由编译器检查
- 返回对象的拷贝由用户控制
我不喜欢方法 B，因为:
- 需要两个难以阅读的 const_casts
这是两种情况的(最小)示例代码。
/**
* summary:
* Two classes with an overloaded method which is
* guaranteed (by contract) not to change any
* internal part of the class. However, there is a
* version of this method that will return a non-const
* reference to an internal object, allowing the user
* to modify it. Don't worry about why I would ever
* want to do this, though if you want a real-world
* example, think about std::vector<>::front()
*
* The difference between A and B can be summarized
* as follows. In both cases, the second method merely
* calls the first, wrapped with the needed
* const_cast's
*
* struct A {
* int& get();
* int get() const;
* };
*
* struct B {
* const int& get() const;
* int& get();
* };
*
**/
struct A
{
int _val;
A() : _val(7) {};
// non-const reference returned here
// by a non-const method
int& get()
{
// maybe lots of calculations that you do not
// wish to be duplicated in the const version
// of this method...
return _val;
}
// const version of get() this time returning
// a copy of the object returned
int get() const
{
// CONST-CAST!!?? SURE.
return const_cast<A*>(this)->get();
}
// example of const method calling the
// overloaded get() method
int deep_get() const
{
// gets a copy and makes
// a copy when returned
// probably optimized away by compiler
return this->get();
}
};
struct B
{
int _val;
B() : _val(7) {};
// const reference returned here
// by a const method
const int& get() const
{
// maybe lots of calculations that you do not
// wish to be duplicated in the non-const
// version of this method...
return _val;
}
// non-const version of get() this time returning
// a copy of the object returned
int& get()
{
// CONST-CAST!? TWO OF THEM!!?? WHY NOT...
return const_cast<int&>(const_cast<const B*>(this)->get());
}
// example of const method calling the
// overloaded get() method
int deep_get() const
{
// gets reference and makes
// a copy when returned
return this->get();
}
};
int main()
{
A a;
a.get() = 8; // call non-const method
a.deep_get(); // indirectly call const method
B b;
b.get() = 8; // call non-const method
b.deep_get(); // indirectly call const method
}