#ifdef _DEBUG的问题

先看MSDN里的解释：
Remarks
Assists in finding memory leaks. You can use DEBUG_NEW everywhere in your program that you would ordinarily use the new operator to allocate heap storage.
In debug mode (when the _DEBUG symbol is defined), DEBUG_NEW keeps track of the filename and line number for each object that it allocates. Then, when you use the CMemoryState::DumpAllObjectsSince member function, each object allocated with DEBUG_NEW is shown with the filename and line number where it was allocated.
To use DEBUG_NEW, insert the following directive into your source files:
#define new DEBUG_NEW
Once you insert this directive, the preprocessor will insert DEBUG_NEW wherever you use new, and MFC does the rest. When you compile a release version of your program, DEBUG_NEW resolves to a simple new operation, and the filename and line number information is not generated.

这样就很清楚了，当在debug模式下时，我们分配内存时的new被替换成DEBUG_NEW，而这个DEBUG_NEW不仅要传入内存块的大小，还要传入源文件名和行号，这就有个