Question

I’ve heard that the static_cast function should be preferred to C-style or simple function-style casting. Is this true? Why?

Answer 1

The main reason is that classic C casts make no distinction between what we call static_cast<>(), reinterpret_cast<>(), const_cast<>(), and dynamic_cast<>(). These four things are completely different.

A static_cast<>() is usually safe. There is a valid conversion in the language, or an appropriate constructor that makes it possible. The only time it’s a bit risky is when you cast down to an inherited class; you must make sure that the object is actually the descendant that you claim it is, by means external to the language (like a flag in the object). A dynamic_cast<>() is safe as long as the result is checked (pointer) or a possible exception is taken into account (reference).

A reinterpret_cast<>() (or a const_cast<>()) on the other hand is always dangerous. You tell the compiler: ‘trust me: I know this doesn’t look like a foo (this looks as if it isn’t mutable), but it is’.

The first problem is that it’s almost impossible to tell which one will occur in a C-style cast without looking at large and disperse pieces of code and knowing all the rules.

Let’s assume these:

class CDerivedClass : public CMyBase {...}; class CMyOtherStuff {...} ;  CMyBase  *pSomething; // filled somewhere 

Now, these two are compiled the same way:

CDerivedClass *pMyObject; pMyObject = static_cast<CDerivedClass*>(pSomething); // Safe; as long as we checked  pMyObject = (CDerivedClass*)(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 C-style casts. 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.