Which is efficient to use #pragma once or #ifndef #endif?

To avoid multiple includes of a header file, one of my friend suggested the following way

  #ifndef _INTERFACEMESSAGE_HPP
  #define _INTERFACEMESSAGE_HPP
  class CInterfaceMessage
  {
     / /Declaration of class goes here
  //i.e declaration of member variables and methods 
   private:
   int m_nCount;
     CString m_cStrMessage;
    public:
    CString foo(int);
 }
   #endif

where _INTERFACEMESSAGE_HPP is just an identifier

but when i declare a class using visual studio 2005 IDE I get a statement as #pragma once at the starting of the class definition when i took the help of msdn to find the purpose of #pragma once it gave me the following explanation

"Specifies that the file will be included (opened) only once by the compiler when compiling a source code file. "

Someone please tell which is the right approach?, if both are correct then what is the difference? is one appro开发者_如何学Cach is better than the other?

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gcc has pragma once as deprecated. You should use the standard include guards. All pragma directives are by definition implementation defined. So, if you want portability, don't use them.

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Pragmas are compiler-specific, so I'd use #ifndef.

Preprocessor directives are resolved during (actually, before) compilation, so they do not make a difference in runtime except maybe for compile time.

However, you will never notice a difference in compile time from these two alternatives unless you use them several thousand times I guess.

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The first approach is the generic approach that works with all compilers and is also the older one around. The #pragma once approach is compiler specific.