Now that we have std::array what uses are left for C-style arrays?

Question

std::array is vastly superior to the C arrays. And even if I want to interoperate with legacy code, I can just use std::array::data(). Is there any reason I would ever want an old-school array?

Answer 1

Unless I've missed something (I've not followed the most recent changes in the standard too closely), most of the uses of C style arrays still remain. std::array does allow static initialization, but it still won't count the initializers for you. And since the only real use of C style arrays before std::array was for statically initialized tables along the lines of:

MyStruct const table[] =
{
    { something1, otherthing1 },
    //  ...
};

using the usual begin and end template functions (adopted in C++11) to iterate over them. Without ever mentionning the size, which the compiler determines from the number of initializers.

EDIT: Another thing I forgot: string literals are still C style arrays; i.e. with type char[]. I don't think that anyone would exclude using string literals just because we have std::array.

Answer 2

No. To, uh, put it bluntly. And in 30 characters.

Of course, you need C arrays to implement std::array, but that's not really a reason that a user would ever want C arrays. In addition, no, std::array is not less performant than a C array, and has an option for a bounds-checked access. And finally, it is completely reasonable for any C++ program to depend on the Standard library- that's kind of the point of it being Standard- and if you don't have access to a Standard library, then your compiler is non-conformant and the question is tagged "C++", not "C++ and those not-C++ things that miss out half the specification because they felt it inappropriate.".

Answer 3

Seems like using multi-dimensional arrays is easier with C arrays than std::array. For instance,

char c_arr[5][6][7];

as opposed to

std::array<std::array<std::array<char, 7>, 6>, 5> cpp_arr;

Also due to the automatic decay property of C arrays, c_arr[i] in the above example will decay to a pointer and you just need to pass the remaining dimensions as two more parameters. My point is it c_arr is not expensive to copy. However, cpp_arr[i] will be very costly to copy.

Answer 4

As Sumant said, multi-dimensional arrays are a lot easier to use with built in C-arrays than with std::array.

When nested, std::array can become very hard to read and unnecessarily verbose.

For example:

std::array<std::array<int, 3>, 3> arr1; 

compared to

char c_arr[3][3]; 

Also, note that begin(), end() and size() all return meaningless values when you nest std::array.

For these reasons I've created my own fixed size multidimensional array containers, array_2d and array_3d. They are analogous to std::array but for multidimensional arrays of 2 and 3 dimensions. They are safer and have no worse performance than built-in multidimensional arrays. I didn't include a container for multidimensional arrays with dimensions greater than 3 as they are uncommon. In C++0x a variadic template version could be made which supports an arbitrary number of dimensions.

An example of the two-dimensional variant:

//Create an array 3 x 5 (Notice the extra pair of braces) 

fsma::array_2d <double, 3, 5> my2darr = {{
    { 32.19, 47.29, 31.99, 19.11, 11.19},
    { 11.29, 22.49, 33.47, 17.29, 5.01 },
    { 41.97, 22.09, 9.76, 22.55, 6.22 }
}};

Full documentation is available here:

http://fsma.googlecode.com/files/fsma.html

You can download the library here:

http://fsma.googlecode.com/files/fsma.zip

Answer 5

The C-style arrays that are available in C++ are actually much less versatile than the real C-arrays. The difference is, that in C, array types can have runtime sizes. The following is valid C code, but it can neither be expressed with C++ C-style arrays nor with the C++ array<> types:

void foo(int bar) {
    double tempArray[bar];
    //Do something with the bar elements in tempArray.
}

In C++, you would have to allocate the temporary array on the heap:

void foo(int bar) {
    double* tempArray = new double[bar];
    //Do something with the bar elements behind tempArray.
    delete[] tempArray;
}

This cannot be achieved with std::array<>, because bar is not known at compile time, it requires the use of either C-style arrays in C++ or of std::vector<>.

---

While the first example could relatively easily be expressed in C++ (albeit requiring new[] and delete[]), the following cannot be achieved in C++ without std::vector<>:

void smoothImage(int width, int height, int (*pixels)[width]) {
    int (*copy)[width] = malloc(height*sizeof(*copy));
    memcpy(copy, pixels, height*sizeof(*copy));
    for(y = height; y--; ) {
        for(x = width; x--; ) {
            pixels[y][x] = //compute smoothed value based on data around copy[y][x]
        }
    }
    free(copy);
}

The point is, that the pointers to the line arrays int (*)[width] cannot use a runtime width in C++, which makes any image manipulation code much more complicated in C++ than it is in C. A typical C++ implementation of the image manipulation example would look like this:

void smoothImage(int width, int height, int* pixels) {
    int* copy = new int[height*width];
    memcpy(copy, pixels, height*width*sizeof(*copy));
    for(y = height; y--; ) {
        for(x = width; x--; ) {
            pixels[y*width + x] = //compute smoothed value based on data around copy[y*width + x]
        }
    }
    delete[] copy;
}

This code does precisely the same calculations as the C code above, but it needs to perform the index computation by hand wherever the indices are used. For the 2D case, this is still feasible (even though it comes with a lot of opportunities to get the index calculation wrong). It gets really nasty in the 3D case, though.

I like writing code in C++. But whenever I need to manipulate multidimensional data, I really ask myself whether I should move that part of the code to C.

Answer 6

May be the std::array is not slow. But I did some benchmarking using simple store and read from the std::array; See the below benchmark results (on W8.1, VS2013 Update 4):

ARR_SIZE: 100 * 1000
Avrg = Tick / ARR_SIZE;

test_arr_without_init
==>VMem: 5.15Mb
==>PMem: 8.94Mb
==>Tick: 3132
==>Avrg: 0.03132
test_arr_with_init_array_at
==>VMem: 5.16Mb
==>PMem: 8.98Mb
==>Tick: 925
==>Avrg: 0.00925
test_arr_with_array_at
==>VMem: 5.16Mb
==>PMem: 8.97Mb
==>Tick: 769
==>Avrg: 0.00769
test_c_arr_without_init
==>VMem: 5.16Mb
==>PMem: 8.94Mb
==>Tick: 358
==>Avrg: 0.00358
test_c_arr_with_init
==>VMem: 5.16Mb
==>PMem: 8.94Mb
==>Tick: 305
==>Avrg: 0.00305

According to the negative marks, the code I used is in the pastebin (link)

The benchmark class code is here;

I don't know a lot about benchmarkings... My code may be flawed

Answer 7

1. to implement something like std::array
2. if you don't want to use the STL or can't
3. For performance