这是严格的混叠规则，可以在c++ 03标准的3.10节中找到(其他答案提供了很好的解释，但没有一个提供了规则本身):

If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined: the dynamic type of the object, a cv-qualified version of the dynamic type of the object, a type that is the signed or unsigned type corresponding to the dynamic type of the object, a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), a type that is a (possibly cv-qualified) base class type of the dynamic type of the object, a char or unsigned char type.

c++ 11和c++ 14的措辞(强调更改):

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined: the dynamic type of the object, a cv-qualified version of the dynamic type of the object, a type similar (as defined in 4.4) to the dynamic type of the object, a type that is the signed or unsigned type corresponding to the dynamic type of the object, a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union), a type that is a (possibly cv-qualified) base class type of the dynamic type of the object, a char or unsigned char type.

有两个变化很小:glvalue代替了lvalue，并澄清了聚合/并集的情况。

第三个变化提供了更强的保证(放宽强混叠规则):类似类型的新概念现在可以安全地进行混叠。

还有C的措辞(C99;Iso / iec 9899:1999 6.5/7;在ISO/IEC 9899:2011§6.5¶7中使用了完全相同的措辞:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types 73) or 88): a type compatible with the effective type of the object, a qualified version of a type compatible with the effective type of the object, a type that is the signed or unsigned type corresponding to the effective type of the object, a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object, an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or a character type. 73) or 88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.

通过指针强制转换(而不是使用联合)的类型双关是打破严格混叠的一个主要例子。

这是严格的混叠规则，可以在c++ 03标准的3.10节中找到(其他答案提供了很好的解释，但没有一个提供了规则本身):

If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined: the dynamic type of the object, a cv-qualified version of the dynamic type of the object, a type that is the signed or unsigned type corresponding to the dynamic type of the object, a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), a type that is a (possibly cv-qualified) base class type of the dynamic type of the object, a char or unsigned char type.

c++ 11和c++ 14的措辞(强调更改):

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined: the dynamic type of the object, a cv-qualified version of the dynamic type of the object, a type similar (as defined in 4.4) to the dynamic type of the object, a type that is the signed or unsigned type corresponding to the dynamic type of the object, a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union), a type that is a (possibly cv-qualified) base class type of the dynamic type of the object, a char or unsigned char type.

有两个变化很小:glvalue代替了lvalue，并澄清了聚合/并集的情况。

第三个变化提供了更强的保证(放宽强混叠规则):类似类型的新概念现在可以安全地进行混叠。

还有C的措辞(C99;Iso / iec 9899:1999 6.5/7;在ISO/IEC 9899:2011§6.5¶7中使用了完全相同的措辞:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types 73) or 88): a type compatible with the effective type of the object, a qualified version of a type compatible with the effective type of the object, a type that is the signed or unsigned type corresponding to the effective type of the object, a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object, an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or a character type. 73) or 88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.

我找到的最好的解释是Mike Acton的《Understanding Strict Aliasing》。本文主要关注PS3的开发，但这基本上只是GCC的工作。

摘自文章:

严格混叠是C(或c++)编译器的一个假设，即指向不同类型对象的指针的解引用永远不会指向相同的内存位置(即相互混叠)。

所以基本上，如果你有一个int*指向一些包含int型的内存，然后你把一个float*指向那个内存，并把它用作浮点数，你就违反了规则。如果你的代码不尊重这一点，那么编译器的优化器很可能会破坏你的代码。

该规则的例外是一个char*，它被允许指向任何类型。

严格的混叠是不允许不同的指针类型指向相同的数据。

本文将帮助您全面详细地理解这个问题。

严格的混叠不只是指指针，它也影响引用，我为boost开发者wiki写了一篇关于它的论文，它很受欢迎，我把它变成了我的咨询网站上的一个页面。它完全解释了它是什么，为什么它让人们如此困惑，以及如何应对它。严格的混叠白皮书。它特别解释了为什么联合对于c++来说是危险的行为，以及为什么使用memcpy是唯一可以在C和c++之间移植的修复程序。希望这对你有帮助。