What's the difference between these two declarations?

struct str1 { ... } ;
typedef struct { ... } str2 ;

Ans : The first form declares a structure tag whereas the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type -- its users don't necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.

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Why is it necessary to use a reference in the argument to the copy constructor?

Ans : If we pass the copy constructor the argument by value, its copy would get constructed using the copy constructor. This means the copy constructor would call itself to make this copy. This process would go on and on until the compiler runs out of memory. This can be explained with the help of following example:

class sample
{

int i ;

public :

sample ( sample p ) 
{

i = p.i ;

}

} ;

void main( )
{

sample s ;
sample s1 ( s ) ;

}

While executing the statement sample s1 ( s ), the copy constructor would get called. As the copy construct here accepts a value, the value of s would be passed which would get collected in p. We can think of this statement as sample p = s. Here p is getting created and initialized. Means again the copy constructor would get called. This would result into recursive calls.Hence we must use a reference as an argument in a copy constructor. 

Accessing Objects from Multiple Threads...

MFC objects are thread-safe at the class level but not at the object level, due to size or the performance reasons. This means that we can have two separate threads manipulating two different CString objects, but not two threads manipulating the same CString object. However, if we want to have multiple threads manipulating the same object, protect such access with appropriate Win32 synchronization mechanisms, such as critical sections. 

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File and FileInfo classes...

.NET offers two classes for file operations - the File class and the FileInfo class. Both these classes are defined in the System::IO namespace hence we must declare the System::IO namespace in the program that uses File or FileInfo class. 

The File class is derived from the Object class. It contains only static methods. Since static methods can be called using the class name itself we are not allowed to create an object of the File class. The FileInfo class is derived from FileSystemInfo class. The FileSystemInfo class is derived from the MarshalByRefObject class which in turn is derived from the Object class.