Dynamic array/ja

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動的配列は、 FreePascal ( と Delphi) のとても便利な機能です。それは、[[Array/ja|配列]型ととてもよく似ていますが、プログラムの実行プログラマーにより高い柔軟性を許します。since the number of elements does not need to be known until program execution.

宣言部は、The declaration part is just as simple as for the array type:

var
  ...
  MyVariable : array of type;
  ...

要素の数は、 SetLength 宣言を挿入することで、プログラムの実行中に、いつでも必要に応じて設定することができます。:

begin
  ...
  SetLength(MyVariable, ItsNewLength);
  ...
end

You can put as many SetLength statements as you want in your program in order to expand or truncate an array but you must put at least one statement before you can use the variable for the first time.

個々の要素へのアクセスは、以下のようにして行います。:

...
SetLength(MyVariable,19);
...
MyVariable[18] := 123;
...
MyOtherVariable := MyVariable[0];
...
WriteLn('MyVariable has ', Length(MyVariable), ' elements');  {should be 19}
...
WriteLn('Its range is ', Low(MyVariable), ' to ', High(MyVariable)); {should be 0 to 18}
...

動的配列のインデックスは、０「ゼロ」が基底となります。すなわち、範囲は 0 から (Length-1)までとなります。これを １「イチ」 を基底にすることはできません。

実際には、Actually, 動的配列はポインタへの自動的な参照のポインタです。dynamic arrays are pointers with automatic dereferencing. それらは、自動的に nil に初期化されます。They are initialized to nil automatically. これは、 fillchar, sizeof,などの低レベルのルーチンを通して扱われるとき、 MyVariable がポインタ変数として翻訳されることを意味します。 This means, that MyVariable is interpreted as a pointer variable when handed over to low level routines like fillchar, sizeof, etc. しかし、but it is automatically expanded to MyVariable^ when indexing its elements (as in MyVariable[2]) or when handing it over to routines that expect array types.

From a memory management view, dynamic array variables are simple pointers. SetLength allocates and frees memory on the heap as needed. When used in functions or procedures only the pointer is added to the stack. When the procedure exits, the dynamic array variable is removed and the memory is made available again. In fact, the memory management procedures are inserted in the executable program and the result is totally transparent to the programmer.

Assigning nil to a dynamic array variable automatically frees the memory where the pointer pointed to. It's identical to SetLength(MyVariable, 0). This can have a side effect, if the pointer value is not valid for some reason (i.e., if it was read from disk where it was stored from previous program runs). To init such an invalid pointer you have to use FillChar(MyVariable,sizeof(MyVariable), #0).

Although writing to elements of dynamic arrays does not create a new instance of the array (no copy-on-write as it exists for Ansistrings) using SetLength on such arrays does create a copy! So if 2 dynamic array variables point to the same array (one has been assigned to the other) they do not do so after using SetLength on one (or both) of them. After the SetLength() call the two variables are distinct arrays whose elements are independent from each other.

関連情報

• Free Pascal Reference guide: Dynamic arrays
• Example: Multidimensional dynamic array/ja
• Array/ja