reinterpret_cast to void* 不适用于函数指针 [英] reinterpret_cast to void* not working with function pointers

问题描述

我想重新解释将函数指针转换为 void* 变量.函数指针的类型将是 Class* (*)(void*) 类型.

I want to reinterpret cast a function pointer into a void* variable. The type of the function pointer will be of type Class* (*)(void*).

以下是示例代码，

class Test
{
    int a;
};

int main()
{
    Test* *p(void **a);
    void *f=reinterpret_cast<void*>(p);     
}

以上代码适用于 Visual Studio/x86 编译器.但是使用 ARM 编译器，它会出现编译错误.不知道为什么.

The above code works well with Visual Studio/x86 compilers. But with ARM compiler, it gives compilation error. Don't know why.

错误:#694:reinterpret_cast 不能抛弃 const 或其他类型预选赛

Error: #694: reinterpret_cast cannot cast away const or other type qualifiers

我阅读了将函数指针转换为另一种类型

我对以下解释感到担忧.

I was concerned about the below explanation.

在函数指针和常规指针(例如，铸造一个 void(*)(void) 到 void*).功能指针不一定相同大小作为常规指针，因为它们可能包含的一些架构额外的上下文信息.这个可能会在 x86 上正常工作，但是请记住，这是未定义的行为.

Casting between function pointers and regular pointers (e.g. casting a void (*)(void) to a void*). Function pointers aren't necessarily the same size as regular pointers, since on some architectures they might contain extra contextual information. This will probably work ok on x86, but remember that it's undefined behavior.

如何从 void (*)(void*) ->void* 有效，因此至少它在大多数编译器中的编译几乎相同?

How to do such conversions from void (*)(void*) -> void* effectively so that atleast it compiles almost the same in most of the compilers ?

推荐答案

reinterpret_cast 不能用于将指向函数的指针转换为 void*.尽管静态、重新解释和 const 类型转换的组合不允许 C 类型转换可以做一些额外的事情，但这种转换不是其中之一.

reinterpret_cast can't be used to cast a pointer to function to a void*. While there are a few additional things that a C cast can do which aren't allowed by combination of static, reinterpret and const casts, that conversion is not one of them.

在 C 中允许转换，但它的行为没有定义(即，即使往返也不能保证工作).

In C the cast is allowed, but it's behavior isn't defined (i.e. even round trip isn't guaranteed to work).

某些 POSIX 函数需要进行转换才能非常有用.

Some POSIX functions need the conversion to be well useful.

我在这里玩过几个编译器:

I've played with several compilers I've here:

• 没有阻止 C 转换，即使在最高一致性模式下也是如此.有些会根据警告和一致性级别发出警告，而其他人则无论我尝试过什么都不会发出警告.
• 即使在更宽松的级别，reinterpret_cast 也是一些编译器的错误，而其他编译器在所有情况下都接受它而从未发出警告.

在 C++0X 的最新可用草案中，有条件地支持函数指针和对象指针之间的 reinterpret_cast.

In the last available draft for C++0X, the reinterpret_cast between function pointers and objects pointers is conditionally supported.

请注意，这是否有意义将更多地取决于目标而不是编译器:像 gcc 这样的可移植编译器将具有由目标架构和可能的 ABI 强加的行为.

Note that if that make sense or not will depend on the target more than the compiler: a portable compiler like gcc will have a behavior imposed by the target architecture and possibly ABI.

正如其他人所说，

Test* *p(void **a);

定义一个函数，而不是一个函数指针.但是指向函数隐式转换的函数是为reinterpret_cast的参数做的，所以reinterpret_cast得到的是一个Test**(*p)(void**a).

defines a function, not a pointer to function. But the function to pointer to function implicit conversion is made for the argument to reinterpret_cast, so what reinterpret_cast get is a Test** (*p)(void** a).

感谢 Richard 让我更深入地重新审视这个问题(为了记录，我错误地认为指向函数的指针指向指向对象的指针是一种情况，其中 C 强制转换允许未经 C++ 强制转换的组合).

Thanks to Richard which makes me revisit the issue more in depth (for the record, I was mistaken in thinking that the pointer to function to pointer to object was one case where the C cast allowed something not authorized by C++ casts combinations).

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