endianess-check with boost differ compared to result by small code

Hi All I am doing some preliminary checking to see whether my system is big-endian or little-endian. In LInux it should be little-endian by default, but I just wanted to double check. I used 2 appro开发者_如何学JAVAaches

1. using boost endian support
2. using some code I found in the web

I used the following static assertion

BOOST_STATIC_ASSERT(!BIG_ENDIAN);

that fails during compile time, so I though..mmhh...is my system big endian? This is the error I have

error: invalid application of ‘sizeof’ to incomplete type 
boost::STATIC_ASSERTION_FAILURE<false>’ 

If I do a test using some code like the one below, it confirm that the system is little-endian. Do you know what I am doing wrong and it I am using Boost macro correctly?

bool is_big_endian_v3(){

    long x = 0x34333231;
    char *y = (char *) &x;

    if(std::strncmp(y,"1234",4)){
        printf("Big Endian");
        return true;
    }else{
        printf("Little Endian");
        return false;
    }
    std::runtime_error ex("I cannot be here");
    throw ex;

}

The Boost library has changed and the code in the previous answer now (July 2019) generates deprecation messages and suggests the following:

#include <boost/predef/other/endian.h>
#include <stdlib.h>
#include <iostream>

int main()
{
#if BOOST_ENDIAN_BIG_BYTE
    std::cout << "Big endian." << std::endl;
#elif BOOST_ENDIAN_LITTLE_BYTE
    std::cout << "Little endian." << std::endl;
#else
    std::cout << "Unknown endian." << std::endl;
#endif
    exit(EXIT_SUCCESS);
}

BIG_ENDIAN is not defined by Boost. If you look at the file

#include <boost/detail/endian.hpp>

you'll see that the macros defined there are BOOST_BIG_ENDIAN, BOOST_LITTLE_ENDIAN, or BOOST_PDP_ENDIAN. That means you need to revise your check to be:

BOOST_STATIC_ASSERT(!defined(BOOST_BIG_ENDIAN)); 

or, better:

BOOST_STATIC_ASSERT(defined(BOOST_LITTLE_ENDIAN)); 

Edit:

The above is not macro expanded as I expected, so I'd suggest to use

#if !defined(BOOST_BIG_ENDIAN) 
BOOST_STATIC_ASSERT(false);
#endif

instead. Sorry for the confusion.