c++ - casting object addresses to char ptrs then using pointer math on them -


according effective c++, "casting object addresses char* pointers , using pointer arithemetic on them yields undefined behavior."

is true plain-old-data? example in template function wrote long ago print bits of object. works splendidly on x86, but... portable?

#include <iostream>  template< typename type > void printbits( type data ) {     unsigned char *c = reinterpret_cast<unsigned char *>(&data);      std::size_t = sizeof(data);     std::size_t b;      while ( i>0 ) {         i--;         b=8;         while ( b > 0 ) {             b--;             std::cout << ( ( c[i] & (1<<b) ) ? '1' : '0' );         }     }     std::cout << "\n"; }  int main ( void ) {     unsigned int f = 0xf0f0f0f0;     printbits<unsigned int>( f );     return 0; }

it not portable. if stick fundamental types, there endianness , there sizeof, function print different results on big-endian machines, or on machines sizeof(int) 16 or 64. issue not pods fundamental types, structs may pod, too.

pod struct members may have internal paddings according implementation-defined alignment rules. if pass pod struct:

struct paddedpod { char c; int i; }

your code print contents of padding, too. , padding different on same compiler different pragmas , options.

on other side, maybe it's wanted.

so, it's not portable, it's not ub. there standard guarantees: can copy pods , array of char or unsigned char, , result of copying via char buffer hold original value. implies can safely traverse array, function safe. nobody guarantees array (or object representation) of objects same type , value same on different computers.

btw, couldn't find passage in effective c++. quote it, pls? say, if part of code contains lots of #ifdef thiscompilerversion, makes sense go all-nonstandard , use hacks lead undefined behavior, work intended on compiler version pragmas , options. in sense, yes, casting char * leads ub.


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