CODEWITHSUNDEEP
c++
olzha bala
olzha bala

Reputation: 173

static_cast<> and unrelated class concersion

I have the following 2 classes:

 class B; 
 class A
 {
     public:
     A();
     operator B() const;
  };
  class B
  {
     public:
     B2();
  };

here, A defines implicit conversion operator to B class. Then C++ reference says the following " If an implicit conversion sequence from new_type to the type of expression exists, that does not include lvalue-to-rvalue, array-to-pointer, function-to-pointer, null pointer, null member pointer, or boolean conversion, then static_cast can perform the inverse of that implicit conversion". It means that the following is to be compiled

  A a;
  B b=a;
  A a1=static_cast<A> (b);

but Xcode gives an error message

Upvotes: 1

Views: 378

Answers (1)

Brian Bi
Brian Bi

Reputation: 119144

Your C++ reference is being imprecise. The Standard (C++14 §5.2.9/7) says,

The inverse of any standard conversion sequence (Clause 4) not containing an lvalue-to-rvalue (4.1), array-to-pointer (4.2), function-to-pointer (4.3), null pointer (4.10), null member pointer (4.11), or boolean (4.12) conversion, can be performed explicitly using static_cast.

Notice that it says standard conversion sequence. static_cast cannot perform the inverse of a user-defined conversion. It is also not hard to see why this is; if static_cast were required to perform the inverse of user-defined conversion sequences, it would have to have the ability to reverse arbitrary algorithms. For example, if A holds a pair of integers and B holds a single integer, and A::operator B() multiplies together the two integers, then the inverse would have to factor the integer in B...

Upvotes: 5

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