Error when assigning Array in Constructor

Question

Here's a weird issue I'm facing - probably something ultra-basic given my rusty C++ skills but I'm still perplexed :

I've got a class
We've also got an array of unsigned long longs in this class - let's call that arr

My Class Interface :

typedef unsigned long long U64;

class DQClass
{
    public:
        DQClass (void);
        virtual ~DQClass (void);

        U64 arr[12];
};

Now as for the implementation...

Test 1 (This works) :

DQClass::DQClass (void)
{
        this->arr[0] = 0x8100000000000000ULL;
        this->arr[1] = 0x4200000000000000ULL;

        // and so on..
}

Test 2 (This doesn't) :

DQClass::DQClass (void)
{
    this->arr = 
    {
        0x8100000000000000ULL,
        0x4200000000000000ULL,
        0x2400000000000000ULL,
        0x1000000000000000ULL,
        0x0800000000000000ULL,
        0x00FF000000000000ULL,
        FLIPV(0x8100000000000000ULL),
        FLIPV(0x4200000000000000ULL),
        FLIPV(0x2400000000000000ULL),
        FLIPV(0x1000000000000000ULL),
        FLIPV(0x0800000000000000ULL),
        FLIPV(0x00FF000000000000ULL)
    };
}

Error :

dqclass.cpp: In constructor ‘DQClass::DQClass()’:
dqclass.cpp:28: error: expected primary-expression before ‘{’ token
dqclass.cpp:28: error: expected `;' before ‘{’ token

Why isn't this working? Shouldn't it be working in the same fashion as, e.g. U64 someArr[12] = {0,1,2,3,4,5,6,7,8,9,10,11} would?

Any ideas?

@NahumLitvin I would be really surprised if something like that worked... And nope, it doesn't. — Dr.Kameleon
– Dr.Kameleon, Commented Dec 13, 2012 at 8:21
Arrays are broken in C++ (because they are broken in C). That's why things like std::vector and std::array were invented. — Benjamin Lindley
– Benjamin Lindley, Commented Dec 13, 2012 at 8:23

Pubby · Accepted Answer · 2012-12-13 08:18:45Z

6

Arrays can't be assigned like that (or any other way), only initialized:

 // sorry for bad formatting
DQClass::DQClass (void)
: arr(
    {
        0x8100000000000000ULL,
        0x4200000000000000ULL,
        0x2400000000000000ULL,
        0x1000000000000000ULL,
        0x0800000000000000ULL,
        0x00FF000000000000ULL,
        FLIPV(0x8100000000000000ULL),
        FLIPV(0x4200000000000000ULL),
        FLIPV(0x2400000000000000ULL),
        FLIPV(0x1000000000000000ULL),
        FLIPV(0x0800000000000000ULL),
        FLIPV(0x00FF000000000000ULL)
    }) {
}

Use constructor initialize list.

You could also use std::array:

std::array<U64, 12> arr;

// ...

this->arr = 
    {{
        0x8100000000000000ULL,
        0x4200000000000000ULL,
        0x2400000000000000ULL,
        0x1000000000000000ULL,
        0x0800000000000000ULL,
        0x00FF000000000000ULL,
        FLIPV(0x8100000000000000ULL),
        FLIPV(0x4200000000000000ULL),
        FLIPV(0x2400000000000000ULL),
        FLIPV(0x1000000000000000ULL),
        FLIPV(0x0800000000000000ULL),
        FLIPV(0x00FF000000000000ULL)
    }};

answered Dec 13, 2012 at 8:18

Pubby

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12 Comments

Dr.Kameleon Over a year ago

I'm not really sure what you're trying to say in your first piece of code... Could please explain to me why U64 someArr[12] = {0,1,2,3,4,5,6,7,8,9,10,11} does work?

Pubby Over a year ago

@Dr.Kameleon Initialization is different from assignment. Arrays have silly behavior and so I can't give you a good reason why arrays can't be assigned. Just use std::array for such purposes.

Pubby Over a year ago

@Dr.Kameleon Maybe this will help stackoverflow.com/questions/3437110/…

Dr.Kameleon Over a year ago

OK, here's another interesting issue : I decided to try the std::array way. While trying #include <array>, I'm getting error: array: No such file or directory and ISO C++ forbids declaration of ‘array’ with no type. (Hint : using g++ in Mac OS X 1.6.8)

Pubby Over a year ago

@Dr.Kameleon You need a fairly recent version that supports C++11. You could also try boost::array which is what std::array is based off of.

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