为什么此函数将'const char *'强制转换为'void * const'而不是'const void *'

Question

This is a very stupid example I came up while working with threads in Windows to wrap a const_cast and a static_cast into one

// Somewhere in the OS API
using function_t = void(*)(void *);
void create_thread(function_t function, void *params);

template <typename T, typename U>
T static_const_cast(U ptr) {
  return const_cast<T>(static_cast<const T>(ptr));
}

void thread_proc(const void* name) {
  // ...
}

void test() {
  const char *name = "name";
  void *params = static_const_cast<void *>(name); // Fails
  // void *params = const_cast<void *>(static_cast<const void *>(name));
  // Succeeds (what I want to achieve with static_const_cast)
  create_thread(reinterpret_cast<function_t>(thread_proc), params);
}

but it won't compile on Visual Studio 2017, so I tried gcc and clang on Godbolt thinking it should surely be able to compile, but still refuses to with the error message (from gcc 8.2):

error: invalid static_cast from type 'const char*' to type 'void* const'
                                                           ^^^^^^^^^^^^^

Why does it try to cast to void* const instead of const void* as expected? Is there something obvious I am missing?

Answer 1

Why does this function cast 'const char*' to 'void* const' and not 'const void*'

Because you used void * as the template argument, and when you apply const to that, it becomes void * const . void * is a pointer (to non-const object), and making a pointer const results in a const pointer, not a (non-const) pointer to const object.

You could do something like this to achieve what you wanted:

return const_cast<T>(
    static_cast<std::remove_pointer_t<T> const*>(ptr)
);

However, I recommend instead to use a custom class to wrap the data that you send to the thread. Also, don't cast the function type. Use a function with void* argument, and cast the argument within the function.

struct params_t {
    const char *name;
};

static params_t params {"name"};

create_thread(thread_proc, &params);

No casts needed whatsoever (except obviously in the callback).