为什么我不能转换“的char **'到'为const char * const的*”用C？ [英] Why can't I convert 'char**' to a 'const char* const*' in C?
问题描述
以下code段（正确）给出了一个在C预警和++ C中的错误（使用gcc和放大器; G ++分别与3.4.5和4.2.1版本的测试; MSVC似乎并不关心）：
The following code snippet (correctly) gives a warning in C and an error in C++ (using gcc & g++ respectively, tested with versions 3.4.5 and 4.2.1; MSVC does not seem to care):
char **a;
const char** b = a;
我能理解并接受这一点。结果
C ++的解决这个问题是要改变b，来是一个const char * const的*，这规避常量，正确性（的 C ++ FAQ ）结果
char **a;
const char* const* b = a;
然而，在纯C，修正版本（中使用const char * const的*）还给出了一个警告，我不明白为什么。
有没有办法来解决这个问题，而无需使用强制转换？
However, in pure C, the corrected version (using const char * const *) still gives a warning, and I don't understand why. Is there a way to get around this without using a cast?
要澄清：结果
1）为什么这个产生在C警告？它应该是完全const的安全，和C ++编译器似乎认识到它本身。结果
2）什么是去接受这个字符**作为参数一边说（和具有强制执行的编译器），我不会修改它指向字符的正确方法是什么？
例如，如果我想编写一个函数：
To clarify:
1) Why does this generate a warning in C? It should be entirely const-safe, and the C++ compiler seems to recognize it as such.
2) What is the correct way to go about accepting this char** as a parameter while saying (and having the compiler enforce) that I will not be modifying the characters it points to?
For example, if I wanted to write a function:
void f(const char* const* in) {
// Only reads the data from in, does not write to it
}
和我想调用它在一个char **，什么是正确的类型参数？
And I wanted to invoke it on a char**, what would be the correct type for the parameter?
编辑：
谢谢那些谁回应，特别是针对谁的问题和/或跟进我的反应。
Thank you to those who have responded, particularly those who addressed the question and/or followed up on my responses.
我已经接受了答案我想做的事情不能没有投来完成，不管它是否应该是可能的。
I've accepted the answer that what I want to do cannot be done without a cast, regardless of whether or not it should be possible.
推荐答案
我在几年前就有这种问题，这惹恼我没有尽头。
I had this same problem a few years ago and it irked me to no end.
在C中的规则更简单地说（即他们没有列出像字符转换** 到为const char * const的异常* ）。 Consequenlty，它只是不允许的。随着C ++标准，其中包括更多的规则允许这样的情况下。
The rules in C are more simply stated (i.e. they don't list exceptions like converting char** to const char*const*). Consequenlty, it's just not allowed. With the C++ standard, they included more rules to allow cases like this.
在最后，它只是在C标准的问题。我希望下一个标准（或技术报告），将解决这个问题。
In the end, it's just a problem in the C standard. I hope the next standard (or technical report) will address this.
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