CODEWITHSUNDEEP
c++castingstructarrays
Yochai Timmer
Yochai Timmer

Reputation: 49231

How can i cast a struct to a char[] array?

I have a variable of type Blah.

I want to cast it to char[sizeof(blah)], without copying.
I need the type cast to be strong enough to instantiate a template that expects char[N].

I've tried many things, but i can't quite get it.
I want something like this to work correctly:

class Blah {
 int a;   
};


template <typename T>
void foo (T& a) 
{ 
    //Not an array
}

template <int N>
void foo (char(&a)[N]) 
{ 
    //an array!
}

Blah b;
foo(b); //not an array
foo((char[sizeofBlah])b); //hopefully treated as an array

Upvotes: 5

Views: 12460

Answers (3)

Konrad Rudolph
Konrad Rudolph

Reputation: 545528

You can’t perform such a cast, that doesn’t make sense. What you can do is get the address of the object and reinterpret the address as a byte address:

char* const buf = reinterpret_cast<char*>(&obj);

That should fulfil your requirements, but beware of using the terminology “cast to char[]” because it obfuscates the actual operation that is taking place.

You can also interpret the address as the starting address of a fixed-sized buffer, of course:

using buffer_t = char[sizeof(Blah)];
buffer_t* pbuf = reinterpret_cast<buffer_t*>(&obj);

But notice that you are still using a pointer to the buffer here.

Upvotes: 10

Eric Postpischil
Eric Postpischil

Reputation: 222362

You can do this with reinterpret_cast<char (&)[sizeof b]>(b), but I do not recommend it.

Upvotes: 6

utnapistim
utnapistim

Reputation: 27365

The cleanest way would be to add it as an operation into the class:

class Blah {
    int a;
public:
    void serialize(char *output) { output[0] = a; /* add others as needed */ }
};

Blah blah;
char buffer[sizeof(Blah)];
blah.serialize(buffer);

This will allow you to explicitly see what's going on and centralize the code in case you need to change it later.

Edit: The serialize interface is not very elegant (or very safe) in my example, but my point is that you should add it as a method.

Upvotes: 1

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