== Quote from Ali Cehreli ([email protected])'s article > I haven't started reading Andrei's chapter on arrays yet. I hope I won't find out that the following behavior is expected. :) > import std.cstream; > void modify(int[] a) > { > a[0] = 1; > a ~= 2; > dout.writefln("During: ", a); > } > void main() > { > int[] a = [ 0 ]; > dout.writefln("Before: ", a); > modify(a); > dout.writefln("After : ", a); > } > The output with dmd 2.035 is > Before: [0] > During: [1,2] > After : [1] > I don't understand arrays. :D > Ali
This is one of those areas where the low-level details of how arrays are implemented arrays leak out. This is unfortunate, but in a close-to-the-metal language it's sometimes a necessary evil. (Dynamic) Arrays are structs that consist of a pointer to the first element and a length. Essentially, the memory being pointed to by the array is passed by reference, but the pointer to the memory and the length of the array are passed by value. While this may seem ridiculous at first, it's a tradeoff that allows for the extremely convenient slicing syntax we have to be implemented efficiently. When you do the a[0] = 1, what you're really doing is: *(a.ptr) = 1; When you do the a ~= 2, what you're really doing is: // Make sure the block of memory pointed to by a.ptr // has enough capacity to be appended to. a.length += 1; *(a.ptr + 1) = 2; Realistically, the only way to understand D arrays and use them effectively is to understand the basics of how they work under the hood. If you try to memorize a bunch of abstract rules, it will seem absurdly confusing.
