safe "struct hack"?

Newsgroup: comp.lang.c++

Subject: safe "struct hack"?

From: Jonathan Lee <jonathan.lee.975@...>

Date: Wed, 14 Aug 2013 06:45:11 -0700 (PDT)



Hello all,

I've have a variation of the struct hack that I think is legal. Wondering if anyone could spot a reason why not. (By struct hack I mean putting a variable length array at the end of a struct, or a struct at the beginning of an array).



Suppose I want to add an int array to the end of



struct header

{

int a;

int b;

double c;

// int arr[]; // I think C99 would do this

}



Then I can make the union



union wrap

{

header hdr;

int arr[1];

}



And use calloc to get some memory



void* p = calloc(20, sizeof(wrap));



The following casts should be legal:

union* u = static_cast<union*>(p);

header* h = static_cast<header*>(u);

int* i = static_cast<int*>(u);



But, then, this should also be legal



int* j = static_cast<int*>(u + 1);



which is an aligned int pointer just past the header

struct. Seeing as how this came from calloc-d, "void"

memory, I think j + 0, j + 1, j + 2, ... are legal

int addresses.



After all, I can legally use



int* q = static_cast<int*>(p);



as an int array. I don't see why j would be any

different.



Can anyone see why this wouldn't be the case?



Thanks in advance,

--Jonathan

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