#pragma pack，模板类型定义和结构对齐 [英] #pragma pack, template typedefs, and struct alignment

问题描述

使用Visual Studio或gcc，如果我有

  #pragma pack（push，16）
 
 typedef std :: map< uint32_t，uint32_t> MyIntMap; 
 
 #pragma pack（pop）
  

然后：

  #pragma pack（push，8）
 
 MyIntMap thisInstance; 
 
 #pragma pack（pop）
  

thisInstance的结构对齐是什么？也就是说，对于typedef'd模板类，编译包是在typedef的地方还是在变量定义的地方生效？如果是后者，有什么方法可以获得跨文件一致对齐的类型？

解决方案

在您的代码中， #pragma pack 将不起作用。只有当它围绕结构或类的定义生效时，它才会做任何事情，而不是围绕typedef或任何东西。它也不会影响变量定义。

你可以在这里看到这个用法： http://msdn.microsoft.com/en-us/library/2e70t5y1（v = VS.100）.aspx

pack在第一个struct，union，或在看到编译指示后的类声明。包对定义没有影响。

Using Visual Studio or gcc, if I've got

#pragma pack(push, 16)

typedef std::map<uint32_t, uint32_t> MyIntMap;

#pragma pack(pop)

then later:

#pragma pack(push, 8)

MyIntMap thisInstance;

#pragma pack(pop)

What is the structure alignment of thisInstance? That is, for a typedef'd template class, does pragma pack take effect at the place of the typedef or at the place of a variable definition? If it's the latter, what's a good workaround to get a type with consistent alignment across files?

解决方案

In your code, the #pragma pack will have no effect. It only does anything when it's in effect around the definition of a struct or class, not around a typedef or anything. Neither does it have any effect around that variable definition.

You can see the usage here: http://msdn.microsoft.com/en-us/library/2e70t5y1(v=VS.100).aspx

Specifically:

pack takes effect at the first struct, union, or class declaration after the pragma is seen. pack has no effect on definitions.

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