在c ++ 11中，dynamic_cast是否返回nullptr或0?

问题描述

我要检查dynamic_cast的结果.在c ++ 11(或c ++ 0x，对于支持nullptr的编译器)中，我应该与nullptr或0进行比较吗?

I want to check the result of dynamic_cast. In c++11 (or c++0x, for compilers that support nullptr), should I compare against nullptr or 0?

有关系吗?如果有，为什么?

Does it matter, and if so, why?

结果是否依赖于编译器?

Is the result compiler-dependent?

推荐答案

常量nullptr(类型为nullptr_t)和常量0都将隐式转换为任何指针类型的null值.因此，与任何一个进行比较将是可行的，并且从技术上讲是可以的.顺便说一句，这意味着dynamic_cast都不返回任何一个，它返回特定指针类型的空值.

Both the constant nullptr (which is of type nullptr_t) and the constant 0 will implicitly convert to the null value of any pointer type. So comparing against either one will work and is technically OK. By the way, this means that dynamic_cast return neither one, it returns the null value for the particular pointer type.

最好是养成使用nullptr而不是0的习惯.据我所知，只有真正适当的重载解析才有必要(例如，一个重载占用int而另一个重载占用char*).为了保持一致性，最好避免使用0.

It's probably best to get in the habit of using nullptr rather than 0. As far as I know, it's only really necessary for proper overload resolution (e.g. one overload takes int and another takes char*). For consistency, avoiding 0 will be best.

指针类型的空值"是什么意思?

考虑变量char * ptr.它的类型是(毫不奇怪)char *.但是nullptr的类型是特殊类型nullptr_t.因此，当我们编写ptr = nullptr之类的东西时，必须发生一些技术性的事情

Consider a variable char * ptr. It's type is (unsurprisingly) char *. But the type of nullptr is the special type nullptr_t. So when we write something like ptr = nullptr, some technical things must happen

1. nullptr必须隐式转换为char *.
2. 此转换的结果设置为ptr的新值.

1. nullptr must be implicitly converted to char *.
2. The result of this conversion is set as the new value of ptr.

char *的空值是将nullptr转换为char *的结果.从概念上讲，它仍然是nullptr，但是具有不同的类型(char *).此空值不同于int *或string *的空值或任何其他指针类型.我们倾向于将这些空值视为nullptr(或0)，但每个值实际上都是与不同类型不同的值. (顺便说一下，使用==进行比较时也会发生相同的转换).

The null value for char * is the result of converting nullptr to char *. Conceptually, it's still nullptr, but with a different type (char *). This null value is distinct from the null value of int * or string * or any other pointer type. We tend to think of these null values as just nullptr (or 0), but each one is really a distinct value from a different type. (By the way, the same conversion happens for comparison using ==).

尽管这听起来像是挑剔的细节，但在重载解析中非常重要:

Although this may sound like nitpicking details, it's very important in overload resolution:

void foo(char * ptr) { ... }
void foo(int i) { ... }
void foo(nullptr_t ptr) { ... }

int main()
{
    foo(0); // Calls void foo(int), since 0 is an int
    foo(nullptr); // Calls void foo(nullptr_t), since nullptr is a nullptr_t
    foo(new char('c')); // Calls void foo(char *), since new char('c') is a char*
}

或分配不相关的空值时

char * c_ptr = nullptr; // Okay
int * i_ptr1 = nullptr; // Okay
int * i_ptr2 = c_ptr;   // COMPILER ERROR HERE

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