说说Delphi 10.4的对象释放
<p>随着Delphi 10.4去掉了ARC，统一移动平台与桌面平台的内存管理，那对于释放对象，有什么变化呢？</p><p>先看看10.4的代码：</p>
<div class="cnblogs_code">
<pre><span style="color: rgba(0, 0, 255, 1)">procedure</span><span style="color: rgba(0, 0, 0, 1)"> TObject.Free;
</span><span style="color: rgba(0, 0, 255, 1)">begin</span>
<span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> under ARC, this method isn't actually called since the compiler translates</span><span style="color: rgba(0, 128, 0, 1)">
//</span><span style="color: rgba(0, 128, 0, 1)"> the call to be a mere nil assignment to the instance variable, which then calls _InstClear</span><span style="color: rgba(0, 128, 0, 1)">
{</span><span style="color: rgba(0, 128, 0, 1)">$IFNDEF AUTOREFCOUNT</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">if</span> Self <> <span style="color: rgba(0, 0, 255, 1)">nil</span> <span style="color: rgba(0, 0, 255, 1)">then</span>
<span style="color: rgba(0, 0, 255, 1)">Destroy</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ENDIF</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 0, 255, 1)">procedure</span><span style="color: rgba(0, 0, 0, 1)"> TObject.DisposeOf;
</span><span style="color: rgba(0, 0, 255, 1)">type</span><span style="color: rgba(0, 0, 0, 1)">
TDestructorProc </span>= <span style="color: rgba(0, 0, 255, 1)">procedure</span><span style="color: rgba(0, 0, 0, 1)"> (Instance: Pointer; OuterMost: ShortInt);
</span><span style="color: rgba(0, 0, 255, 1)">begin</span>
<span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$IFDEF AUTOREFCOUNT</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">if</span> Self <> <span style="color: rgba(0, 0, 255, 1)">nil</span> <span style="color: rgba(0, 0, 255, 1)">then</span>
<span style="color: rgba(0, 0, 255, 1)">begin</span><span style="color: rgba(0, 0, 0, 1)">
Self.__ObjAddRef; </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> Ensure the instance remains alive throughout the disposal process</span>
<span style="color: rgba(0, 0, 255, 1)">try</span>
<span style="color: rgba(0, 0, 255, 1)">if</span> __SetDisposed(Self) <span style="color: rgba(0, 0, 255, 1)">then</span>
<span style="color: rgba(0, 0, 255, 1)">begin</span><span style="color: rgba(0, 0, 0, 1)">
_BeforeDestruction(Self, </span><span style="color: rgba(128, 0, 128, 1)">1</span><span style="color: rgba(0, 0, 0, 1)">);
TDestructorProc(PPointer(PByte(PPointer(Self)^) </span>+ vmtDestroy)^)(Self, <span style="color: rgba(128, 0, 128, 1)">0</span><span style="color: rgba(0, 0, 0, 1)">);
</span><span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 0, 255, 1)">finally</span><span style="color: rgba(0, 0, 0, 1)">
Self.__ObjRelease; </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> This will deallocate the instance if the above process cleared all other references.</span>
<span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ELSE</span><span style="color: rgba(0, 128, 0, 1)">}</span><span style="color: rgba(0, 0, 0, 1)">
Free;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ENDIF</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">end</span>;</pre>
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<p>可以清楚的看到，在DisposeOf中，如果没有定义<span style="color: rgba(0, 128, 0, 1)">AUTOREFCOUNT</span>编译变量，则直接调用Free方法。由于去掉ARC，<span style="color: rgba(0, 128, 0, 1)">AUTOREFCOUNT</span>不再定义，所以调用DisposeOf，就是调用Free。现在可以忘记DisposeOf了，<strong>所有平台释放对象，就用Free。</strong></p>
<p>接下来，再看看FreeAndNil方法：</p>
<div class="cnblogs_code">
<pre><span style="color: rgba(0, 0, 255, 1)">procedure</span> <span style="color: rgba(0, 0, 255, 1)">FreeAndNil</span>(<span style="color: rgba(0, 0, 255, 1)">const</span><span style="color: rgba(0, 0, 0, 1)"> Obj: TObject);
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$IF not Defined(AUTOREFCOUNT)</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">var</span><span style="color: rgba(0, 0, 0, 1)">
Temp: TObject;
</span><span style="color: rgba(0, 0, 255, 1)">begin</span><span style="color: rgba(0, 0, 0, 1)">
Temp :</span>=<span style="color: rgba(0, 0, 0, 1)"> Obj;
TObject(Pointer(@Obj)^) :</span>= <span style="color: rgba(0, 0, 255, 1)">nil</span><span style="color: rgba(0, 0, 0, 1)">;
Temp.Free;
</span><span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ELSE</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">begin</span><span style="color: rgba(0, 0, 0, 1)">
Obj :</span>= <span style="color: rgba(0, 0, 255, 1)">nil</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 0, 255, 1)">end</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ENDIF</span><span style="color: rgba(0, 128, 0, 1)">}</span></pre>
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<p>这个方法，同样使用了<span style="color: rgba(0, 128, 0, 1)">AUTOREFCOUNT</span>编译变量。</p>
<p>为了验证移动平台下，是否定义了<span style="color: rgba(0, 128, 0, 1)">AUTOREFCOUNT</span>，笔者做了个测试代码：</p>
<div class="cnblogs_code">
<pre><span style="color: rgba(0, 0, 255, 1)">procedure</span><span style="color: rgba(0, 0, 0, 1)"> TForm1.FormCreate(Sender: TObject);
</span><span style="color: rgba(0, 0, 255, 1)">begin</span>
<span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$IFDEF AUTOREFCOUNT</span><span style="color: rgba(0, 128, 0, 1)">}</span><span style="color: rgba(0, 0, 0, 1)">
Text1.Text:</span>=<span style="color: rgba(128, 0, 0, 1)">'</span><span style="color: rgba(128, 0, 0, 1)">AUTOREFCOUNT</span><span style="color: rgba(128, 0, 0, 1)">'</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ELSE</span><span style="color: rgba(0, 128, 0, 1)">}</span><span style="color: rgba(0, 0, 0, 1)">
Text1.Text:</span>=<span style="color: rgba(128, 0, 0, 1)">'</span><span style="color: rgba(128, 0, 0, 1)">Not Defined AUTOREFCOUNT</span><span style="color: rgba(128, 0, 0, 1)">'</span><span style="color: rgba(0, 0, 0, 1)">;
</span><span style="color: rgba(0, 128, 0, 1)">{</span><span style="color: rgba(0, 128, 0, 1)">$ENDIF</span><span style="color: rgba(0, 128, 0, 1)">}</span>
<span style="color: rgba(0, 0, 255, 1)">end</span>;</pre>
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<p>事实证明，在android运行时，显示<span style="color: rgba(128, 0, 0, 1)">Not Defined AUTOREFCOUNT</span>。没有定义<span style="color: rgba(0, 128, 0, 1)">AUTOREFCOUNT</span>，你可按这个去读上面的代码了！</p>
<p> </p><br><br>
来源：https://www.cnblogs.com/kinglandsoft/p/13151911.html
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