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<h3><span lang=EN-US>60. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>避免在不同的模块中分配和释放内存</span></h3>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>摘要</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>找到东西就把它们放回原位：在一个模块中分配内存而在另一个不同的模块中释放它们会使程序变得脆弱，因为这样会在这些模块间产生不易察觉的远程依赖性。这些模块必须用相同版本的编译器，相同的编译标志（特别是</span><span
lang=EN-US>debug vs. NDEBUG</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），以及相同的标准库实现来编译。实际上，在释放内存时，用来分配内存的模块最好仍在内存中。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>讨论</span><span lang=EN-US><o:p></o:p></span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>库的提供者希望改进库的质量，这样的一个直接结果就是库的不同版本中标准内存分配器使用的内部数据结构和算法可能会变化很大。此外，不同的编译选项（例如：打开和关闭调试功能）也可能在极大程度上改变内存分配器的内部工作方式。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>因此，在跨越不同的模块时——尤其是不能保证会用同一个编译器和相同的编译选项来编译的模块时，不要对内存分配函数（例如：</span><span
lang=EN-US>::operator delete</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>或</span><span
class=SpellE><span lang=EN-US>std::free</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）做任何假设。这种情况经常是不同的模块在同一个</span><span
class=SpellE><span lang=EN-US>makefile</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>中，用相同的选项来编译，但是舒适常常会导致健忘。特别是遇到动态链接库，由多个大型团队分布式开发的大型项目，或者是极具挑战性的模块“热交换”这些情况时，应该高度谨慎以保证在同一个模块或子系统中分配和释放内存。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>要确保内存是由正确的函数释放的，一个不错的方法是使用</span><span
class=SpellE><span lang=EN-US>shared_ptr</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>（参见</span><span
lang=EN-US>[C++TR104]</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）。</span><span
class=SpellE><span lang=EN-US>shared_ptr</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>是一个使用了引用计数的智能指针，它可以在构造的时候记住“删除者”。删除者是一个执行释放操作的仿函数（或直接是一个函数指针）。因为前述仿函数，或函数指针，是</span><span
class=SpellE><span lang=EN-US>shared_ptr</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>对象的状态的一部分，所以分配该对象的模块可以同时指定用于释放的函数，即使释放操作实际发生的位置是在另一个模块中，该函数也可以被正确地调用，不可否认，这是以一点额外的开销为代价的（以这点开销换得正确性是值得，参见第</span><span
lang=EN-US>5</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，</span><span lang=EN-US>6</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，</span><span lang=EN-US>8</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）。当然，原来的模块仍必须保留在内存中。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
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