C ++中的部分const类型转换

Question

Sometimes, when passing or returning structs, it may be needed to make some of the fields const:

struct A
{
   char c;
   int x;
};

struct B
{
   const char c;
   int x;
};

void process(B& b)
{
   if(b.c=='1')
      b.x++;
}

void test()
{
   A a;
   a.c = '1';
   a.x = 0;
   process(reinterpret_cast<B&>(a));
}

Is this partial const typecasting portable and safe enough?

Answer 1

没有这样的技术是不安全的，这是不确定的行为-在不相关的类型之间进行强制转换（即使它们看起来很相似）也不是安全的。

Answer 2

It is safe only for the sample from your code . In general the technique is not safe.

The ac isn't constant in the moment of it's declaration , so Bc will inherit the c value , and the constness of c will be use only to enforce syntax.

The problem would occur if the it was the other way around :

B b;
b.c = '1';
b.x = '0;
process(reinterpret_cast<A&>(b));

The reason would be that bc would be assigned to a constant at the time of declaration, and attempting to change ac could result in undefined behavior because you might end up changing a variable from a read only space.

Answer 3

As pointed out by Wojtek.It is not safe clearly because of casting between 2 different types and shouldn't be used.But just for demonstration purposes as i mentioned in comment we can do chaining of typecasting for this particular case.

1st add constness in A's object and then use reinterpret_cast

struct A
{
   char c;
   int x;
};

struct B
{
   const char c;
   int x;
};

void process(B& b)
{
   if(b.c=='1')
      b.x++;

      cout<<b.x;
}


void  test() {
    // your code goes here
    A a;
   a.c = '1';
   a.x = 3;
   process(reinterpret_cast<B&>(const_cast<A&>(a))); //first add constness to A's data and then use reinterpret_cast

}

it is demonstrated here. http://ideone.com/xnblvi If anyone find any problem please comment.