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# core.internal.array.capacity

This module contains support for controlling dynamic arrays' capacity and length
License:
Distributed under the Boost Software License 1.0. (See accompanying file LICENSE)

Source core/internal/array/capacity.d

pure nothrow @property @trusted size_t `capacity`(T)(T[] `arr`);
  (Property) Gets the current capacity of a slice. The capacity is the size that the slice can grow to before the underlying array must be reallocated or extended.
  If an append must reallocate a slice with no possibility of extension, then 0 is returned. This happens when the slice references a static array, or if another slice references elements past the end of the current slice.

  Note The capacity of a slice may be impacted by operations on other slices.

  Examples:

      //Static array slice: no capacity
      int[4] sarray = [1, 2, 3, 4];
      int[]  slice  = sarray[];
      assert(sarray.capacity == 0);
      //Appending to slice will reallocate to a new array
      slice ~= 5;
      assert(slice.capacity >= 5);

      //Dynamic array slices
      int[] a = [1, 2, 3, 4];
      int[] b = a[1 .. $];
      int[] c = a[1 .. $ - 1];
      debug(SENTINEL) {} else // non-zero capacity very much depends on the array and GC implementation
      {
          assert(a.capacity != 0);
          assert(a.capacity == b.capacity + 1); //both a and b share the same tail
      }
      assert(c.capacity == 0);              //an append to c must relocate c.

pure nothrow @trusted size_t `reserve`(T)(ref T[] `arr`, size_t `newcapacity`);
  Reserves capacity for a slice. The capacity is the size that the slice can grow to before the underlying array must be reallocated or extended.
  Returns:
  The new capacity of the array (which may be larger than the requested capacity).
  Examples:

      //Static array slice: no capacity. Reserve relocates.
      int[4] sarray = [1, 2, 3, 4];
      int[]  slice  = sarray[];
      auto u = slice.reserve(8);
      assert(u >= 8);
      assert(&sarray[0] !is &slice[0]);
      assert(slice.capacity == u);

      //Dynamic array slices
      int[] a = [1, 2, 3, 4];
      a.reserve(8); //prepare a for appending 4 more items
      auto p = &a[0];
      u = a.capacity;
      a ~= [5, 6, 7, 8];
      assert(p == &a[0]);      //a should not have been reallocated
      assert(u == a.capacity); //a should not have been extended

nothrow ref @system inout(T[]) `assumeSafeAppend`(T)(auto ref inout(T[]) `arr`);
  Assume that it is safe to append to this array. Appends made to this array after calling this function may append in place, even if the array was a slice of a larger array to begin with.
  Use this only when it is certain there are no elements in use beyond the array in the memory block. If there are, those elements will be overwritten by appending to this array.

  Warning Calling this function, and then using references to data located after the given array results in undefined behavior.

  Returns:
  The input is returned.
  Examples:

      int[] a = [1, 2, 3, 4];

      // Without assumeSafeAppend. Appending relocates.
      int[] b = a [0 .. 3];
      b ~= 5;
      assert(a.ptr != b.ptr);

      debug(SENTINEL) {} else
      {
          // With assumeSafeAppend. Appending overwrites.
          int[] c = a [0 .. 3];
          c.assumeSafeAppend() ~= 5;
          assert(a.ptr == c.ptr);
      }

template `_d_arraysetlengthTImpl`(Tarr : T[], T)
  Implementation of _d_arraysetlengthT and _d_arraysetlengthTTrace
  pure nothrow @trusted size_t `_d_arraysetlengthT`(return ref scope Tarr `arr`, size_t `newlength`);
    Resize dynamic array
    Parameters:

    Tarr `arr` | the array that will be resized, taken as a reference
    size_t `newlength` | new length of array

    Returns:
    The new length of the array
    Bugs:
    The safety level of this function is faked. It shows itself as @trusted pure nothrow to not break existing code.
  alias `_d_arraysetlengthTTrace` = _d_HookTraceImpl!(Tarr, _d_arraysetlengthT, errorMessage);
    TraceGC wrapper around core.internal.array.core.internal.array.capacity.d_arraysetlengthT.
    Bugs:
    This function template was ported from a much older runtime hook that bypassed safety, purity, and throwabilty checks. To prevent breaking existing code, this function template is temporarily declared @trusted pure nothrow until the implementation can be brought up to modern D expectations.

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