Iterating through array of object pointers in C++ the old way (without range-based for)?

Question

I'm not really versed in C++, but I came to the following code:

BaseManager* allManagers[] =
{
    mColorManager,
    mToolManager,
    mLayerManager,
    mPlaybackManager,
    mViewManager
};

for ( BaseManager* pManager : allManagers )
{
    pManager->setEditor( this );
    pManager->init();
}

I use an older g++, so I cannot use -std=c++11, and I have to use -std=c++0x. Looking at the "old-school" equivalent in error: expected initializer before ‘:’ token, I would have hoped the following would work:

for ( auto it = allManagers.begin(); it != allManagers.end(); ++it )
{
    BaseManager* pManager = *it;
    pManager->setEditor( this );
    pManager->init();
}

... but it fails with:

error: request for member ‘begin’ in ‘allManagers’, which is of non-class type ‘BaseManager* [5]’
error: unable to deduce ‘auto’ from ‘<expression error>’

So I gathered, since this allManagers is apparently just a C array, it is not an object with methods (like .begin) - so in the end, I finally got that piece to compile with:

for ( int i=0 ; i<5 ; i++ )
{
    BaseManager* pManager = allManagers[i];
    pManager->setEditor( this );
    pManager->init();
}

... however, that requires me to write in the array length manually, which I don't like.

So, my question is: what would be the proper way to iterate through such an array of pointers without using a range-based for loop - but also, without having to explicitly enter the hardcoded array length?

The iterator versions below are better, but: `for ( int i = 0; i < (sizeof(allManagers) / sizeof(allManagers[0])); ++i )` is a C-ish way to do it. — Paul Roub, Jan 08 '15 at 23:25

R Sahu · Answer 1 · 2015-01-08T23:34:10.440

6

You can use:

for ( auto it = std::begin(allManagers); it != std::end(allManagers); ++it )
{

However, these are also C++11 features. Some pre C++11 compilers might support them but it is not guaranteed.

Without a C++11 compiler to work with, you can use:

size_t numObjects = sizeof(allManagers)/sizeof(allManagers[0]);
for ( size_t i = 0; i < numObjects; ++i  )
{
   BaseManager* manager = allManagers[i];

edited Jan 08 '15 at 23:34

answered Jan 08 '15 at 23:23

R Sahu

204,454
14
159
270

2

Nice, totally didn't realize that was in the standard library. For OP, too, look here for how to implement them if you're curious: http://stackoverflow.com/questions/5897319/how-to-use-stdsort-to-sort-an-array-in-c – mbgda Jan 08 '15 at 23:26
Thanks @RSahu - I was really looking forward to this, but apparently I have a compiler which is too old, as I got "error: ‘begin’ is not a member of ‘std’" ... – sdaau Jan 08 '15 at 23:28

mbgda · Answer 2 · 2015-01-09T00:02:29.743

2

Either track the array length in a variable or use an std::array or std::vector.

Edit: If you want to create your own ArrayLen() method, you can do this:

template<class T, size_t Size>
static inline size_t ArrayLen(const T(&arr)[Size])
{
    return Size;
}

Then you can do this:

for(size_t i = 0; i < ArrayLen(allManagers); ++i)
{
    BaseManager * pManager = allManagers[i];

edited Jan 09 '15 at 00:02

answered Jan 08 '15 at 23:22

mbgda

787
5
8

Thanks @mbgda - does that mean, that there is no function in "old" c++ to determine the length of a "plain" C array (as in, pseudocode, `int mylen = len(allManagers);`)? – sdaau Jan 08 '15 at 23:31
@sdaau You can get the length of the array using modified code from the page I linked elsewhere, like this: `template size_t ArraySize(const T(&array)[Size]) { return Size; }` – mbgda Jan 08 '15 at 23:34

score 1 · Accepted Answer · edited May 23 '17 at 12:18

1

allManagers is not a container. It is an array of pointers, thus you can not use .begin().

If you want to determine the size of your array, just do this:

int n = sizeof(allManagers)/sizeof(allManagers[0]);

Check this question for more. Notice that your array is in the stack.

edited May 23 '17 at 12:18

Community

1
1

answered Jan 08 '15 at 23:22

gsamaras

71,951
46
188
305

Thanks for confirming that, @G.Samaras – sdaau Jan 08 '15 at 23:32
You are welcome @sdaau. Take a look at my edit. Nice question by the way. – gsamaras Jan 08 '15 at 23:33
1

Thanks again, @G.Samaras - the edit was exactly what I needed; I tried the counted for loop `for ( int i=0 ; i< (sizeof allManagers / sizeof allManagers[0]) ; i++ )` and it compiled just fine! So I'm accepting this answer (even if I wish I could have used the syntax noted by @RSahu). Cheers! – sdaau Jan 08 '15 at 23:37
You are welcome. The compiler will probably do this for you (with the needed optimization flag), but you it would be nice to "cache" the result of a division to a variable (just like in my answer) and put that in the control of for. Otherwise, it will be executed every time you perform the for check! @sdaau – gsamaras Jan 08 '15 at 23:39

Iterating through array of object pointers in C++ the old way (without range-based for)?

3 Answers3