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Rule:  CON#004Checked automatically with code checker

Synopsis:Use the new cast operators (static_cast, const_cast, dynamic_cast, and reinterpret_cast) instead of the C-style casts
Language:C++
Severity Level:1
Category:Conversions


Description:

Let's assume these:

class CMyClass : public CMyBase {...};
class CMyOtherStuff {...} ;

CMyBase  *pSomething; // Filled somewhere

Now, these two are compiled the same way:

CMyClass *pMyObject;
pMyObject = static_cast<CMyClass*>(pSomething); // Safe; as long as we checked

pMyObject = (CMyClass*)(pSomething); // Same as static_cast<>. Safe; as long as we checked, but harder to read

However, let's see this almost identical code:

CMyOtherStuff *pOther;
pOther = static_cast<CMyOtherStuff*>(pSomething); // Compiler error: Can't convert

pOther = (CMyOtherStuff*)(pSomething); // No compiler error. Same as reinterpret_cast<> and it's wrong!!!

As you can see, there is no easy way to distinguish between the two situations without knowing a lot about all the classes involved.

The second problem is that the C-style casts are too hard to locate. In complex expressions it can be very hard to see a C-style cast. It is virtually impossible to write an automated tool that needs to locate C-style casts (for example a search tool) without a full blown C++ compiler front-end. On the other hand, it's easy to search for "static_cast<" or "reinterpret_cast<".

pOther = reinterpret_cast<CMyOtherStuff*>(pSomething);
      // No compiler error.
      // but the presence of a reinterpret_cast<> is 
      // like a Siren with Red Flashing Lights in your code.
      // The mere typing of it should cause you to feel VERY uncomfortable.

That means that, not only are C-style casts more dangerous, but it's a lot harder to find them all to make sure that they are correct.

See also [CON#006] for a discussion about the new cast operators.

Since conversion functions for built-in types such as behave exactly the same as C style casts, these aren't allowed either:

a = int(b); // Wrong, is the same as a = (int) b;

Exception to this rule: Using (void) to cast-away the return value is allowed.



Literature References:
ISC++ Rec. 6.2
StackOverflow Q103512