使用 #pragma Once 或 #ifndef #endif 哪个更有效？
为了避免多次包含头文件，我的一位朋友建议采用以下方法
#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
，其中 _INTERFACEMESSAGE_HPP 只是一个标识符，
但是当我使用 Visual Studio 2005 IDE 声明一个类时，我得到一条语句： #pragma 一次 在类定义的开头 当我在msdn的帮助下找到#pragma的目的时 它给了我以下解释
“指定编译器在编译源代码文件时仅包含（打开）该文件一次。”
有人请告诉我哪种方法是正确的？如果两者都正确，那么有什么区别？一种方法比另一种更好吗？
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 approach is better than the other?
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gcc 有一次 pragma 已弃用。您应该使用标准包含防护。所有编译指令都是根据定义实现来定义的。因此，如果您想要便携性，请不要使用它们。
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.
编译指示是特定于编译器的，因此我会使用#ifndef。
预处理器指令在编译期间（实际上是在编译之前）解析，因此除了编译时之外，它们在运行时不会产生任何影响。
然而，我猜你永远不会注意到这两种替代方案在编译时间上的差异，除非你使用它们几千次。
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.
第一种方法是适用于所有编译器的通用方法，也是较旧的方法。
#pragma Once方法是特定于编译器的。The first approach is the generic approach that works with all compilers and is also the older one around. The
#pragma onceapproach is compiler specific.