Why do we have reinterpret_cast in C++ when two chained static_cast can do its job?

Question

Say I want to cast A* to char* and vice-versa, we have two choices (I mean, many of us think we've two choices, because both seems to work! Hence the confusion!):

struct A {     int age;     char name[128]; };  A a; char *buffer = static_cast<char*>(static_cast<void*>(&a)); //choice 1 char *buffer = reinterpret_cast<char*>(&a); //choice 2 

Both work fine.

//convert back A *pA = static_cast<A*>(static_cast<void*>(buffer)); //choice 1 A *pA = reinterpret_cast<A*>(buffer); //choice 2 

Even this works fine!

So why do we have reinterpret_cast in C++ when two chained static_cast can do its job?

Some of you might think this topic is a duplicate of the previous topics such as listed at the bottom of this post, but it's not. Those topics discuss only theoretically, but none of them gives even a single example demonstrating why reintepret_cast is really needed, and two static_cast would surely fail. I agree, one static_cast would fail. But how about two?

If the syntax of two chained static_cast looks cumbersome, then we can write a function template to make it more programmer-friendly:

template<class To, class From> To any_cast(From v) {     return static_cast<To>(static_cast<void*>(v)); } 

And then we can use this, as:

char *buffer = any_cast<char*>(&a); //choice 1 char *buffer = reinterpret_cast<char*>(&a); //choice 2  //convert back A *pA = any_cast<A*>(buffer); //choice 1 A *pA = reinterpret_cast<A*>(buffer); //choice 2 

Also, see this situation where any_cast can be useful: Proper casting for fstream read and write member functions.

So my question basically is,

• Why do we have reinterpret_cast in C++?
• Please show me even a single example where two chained static_cast would surely fail to do the same job?

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• Which cast to use; static_cast or reinterpret_cast?
• Cast from Void* to TYPE* : static_cast or reinterpret_cast

Answer 1

There are things that reinterpret_cast can do that no sequence of static_casts can do (all from C++03 5.2.10):

• A pointer can be explicitly converted to any integral type large enough to hold it.

• A value of integral type or enumeration type can be explicitly converted to a pointer.

• A pointer to a function can be explicitly converted to a pointer to a function of a different type.

• An rvalue of type "pointer to member of X of type T1" can be explicitly converted to an rvalue of type "pointer to member of Y of type T2" if T1 and T2 are both function types or both object types.

Also, from C++03 9.2/17:

• A pointer to a POD-struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa.

Answer 2

You need reinterpret_cast to get a pointer with a hardcoded address (like here):

int* pointer = reinterpret_cast<int*>( 0x1234 ); 

you might want to have such code to get to some memory-mapped device input-output port.