铸造的Struct到数组 | 铸造的Struct到数组

本文介绍了铸造的Struct到数组的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

这是一个严格走样的问题，如将编译器造成任何优化订单的问题与此有关。

说我在结构XMFLOAT3 （不同于浮动取值href=\"https://msdn.microsoft.com/en-us/library/windows/desktop/microsoft.directx_sdk.reference.xmfloat3%28v=vs.85%29.aspx\"相对=nofollow>这个。）我想转换为浮法* 。这将降落在我的优化麻烦吗？

  XMFLOAT3富= {1.0F，2.0F，3.0F};
自动栏=安培; foo.x;酒吧[2] + = 5.0F;
foo.z + = 5.0F;
COUT＆LT;＆LT; foo.z;

我相信这将始终打印13。但是这个code：

  XMFLOAT3富= {1.0F，2.0F，3.0F};
自动栏= reinter pret_cast＆LT;浮动*＆GT;（安培;富）;酒吧[2] + = 5.0F;
foo.z + = 5.0F;
COUT＆LT;＆LT; foo.z;

我相信这是合法的，因为根据的

Is my understanding of this correct?

解决方案

The reinterpret_cast from XMFLOAT3* to float* is OK, due to:

9.2 [class.mem] paragraph 20:

That means the address of the first member is the address of the struct, and there's no aliasing involved when you access *bar because you're accessing a float through an lvalue of type float, which is fine.

But the cast is also unnecessary, it's equivalent to the first version:

auto bar = &foo.x;

The expression bar[2] is only OK if there is no padding between the members of the struct, or more precisely, if the layout of the data members is the same as an array float[3], in which case 3.9.2 [basic.compound] paragraph 3 says it is OK:

In practice there is no reason that three adjacent non-static data members of the same type would not be laid out identically to an array (and I think the Itanium ABI guarantees it), but to be safe you could add:

 static_assert(sizeof(XMFLOAT3)==sizeof(float[3]),
     "XMFLOAT3 layout must be compatible with float[3]");

Or to be paranoid, or if there are just additional members after z:

 static_assert(offsetof(XMFLOAT3, y)==sizeof(float)
               && offsetof(XMFLOAT3, z)==sizeof(float)*2,
     "XMFLOAT3 layout must be compatible with float[3]");

Yes, it relies on it being a standard-layout class type, and on the order and type of its data members.

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