从技术上讲，在c++中，严格的混叠规则可能永远都不适用。

注意indirection(*运算符)的定义:

一元*运算符执行间接操作:它所对应的表达式 是指向对象类型的指针，还是指向对象类型的指针 函数类型，结果是指向对象或的左值 表达式所指向的函数。

同样来自glvalue的定义

glvalue是一个表达式，其求值决定的标识 一个对象，(…剪)

因此，在任何定义良好的程序跟踪中，glvalue都指向对象。所以所谓的严格混叠规则并不适用。这可能不是设计师想要的。

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.

从技术上讲，在c++中，严格的混叠规则可能永远都不适用。

注意indirection(*运算符)的定义:

一元*运算符执行间接操作:它所对应的表达式 是指向对象类型的指针，还是指向对象类型的指针 函数类型，结果是指向对象或的左值 表达式所指向的函数。

同样来自glvalue的定义

glvalue是一个表达式，其求值决定的标识 一个对象，(…剪)

因此，在任何定义良好的程序跟踪中，glvalue都指向对象。所以所谓的严格混叠规则并不适用。这可能不是设计师想要的。

一个典型的情况下，你遇到严格的混叠问题是当覆盖一个结构(如设备/网络msg)到你的系统的字大小的缓冲区(如uint32_ts或uint16_ts指针)。当你将一个结构叠加到这样的缓冲区上，或者通过指针强制转换将一个缓冲区叠加到这样的结构上时，你很容易违反严格的混叠规则。

在这种设置中，如果我想发送消息到某个对象，我必须有两个不兼容的指针指向同一块内存。然后我可能会天真地编写如下代码:

typedef struct Msg
{
    unsigned int a;
    unsigned int b;
} Msg;

void SendWord(uint32_t);

int main(void)
{
    // Get a 32-bit buffer from the system
    uint32_t* buff = malloc(sizeof(Msg));
    
    // Alias that buffer through message
    Msg* msg = (Msg*)(buff);
    
    // Send a bunch of messages    
    for (int i = 0; i < 10; ++i)
    {
        msg->a = i;
        msg->b = i+1;
        SendWord(buff[0]);
        SendWord(buff[1]);   
    }
}

严格的混叠规则使得这种设置是非法的:对一个指针进行解引用，该指针的混叠对象不是兼容类型或C 2011 6.5第71段允许的其他类型之一，这是未定义的行为。不幸的是，您仍然可以以这种方式编码，可能会得到一些警告，让它编译良好，但在运行代码时却会出现奇怪的意外行为。

(GCC在给出别名警告的能力上似乎有些不一致，有时给我们一个友好的警告，有时则不是。)

To see why this behavior is undefined, we have to think about what the strict aliasing rule buys the compiler. Basically, with this rule, it doesn't have to think about inserting instructions to refresh the contents of buff every run of the loop. Instead, when optimizing, with some annoyingly unenforced assumptions about aliasing, it can omit those instructions, load buff[0] and buff[1] into CPU registers once before the loop is run, and speed up the body of the loop. Before strict aliasing was introduced, the compiler had to live in a state of paranoia that the contents of buff could change by any preceding memory stores. So to get an extra performance edge, and assuming most people don't type-pun pointers, the strict aliasing rule was introduced.

请记住，如果您认为这个示例是虚构的，那么即使您将缓冲区传递给另一个为您执行发送的函数，也可能会发生这种情况。

void SendMessage(uint32_t* buff, size_t size32)
{
    for (int i = 0; i < size32; ++i) 
    {
        SendWord(buff[i]);
    }
}

并重写了之前的循环来利用这个方便的函数

for (int i = 0; i < 10; ++i)
{
    msg->a = i;
    msg->b = i+1;
    SendMessage(buff, 2);
}

The compiler may or may not be able to or smart enough to try to inline SendMessage and it may or may not decide to load or not load buff again. If SendMessage is part of another API that's compiled separately, it probably has instructions to load buff's contents. Then again, maybe you're in C++ and this is some templated header only implementation that the compiler thinks it can inline. Or maybe it's just something you wrote in your .c file for your own convenience. Anyway undefined behavior might still ensue. Even when we know some of what's happening under the hood, it's still a violation of the rule so no well defined behavior is guaranteed. So just by wrapping in a function that takes our word delimited buffer doesn't necessarily help.

我该怎么解决这个问题呢?

Use a union. Most compilers support this without complaining about strict aliasing. This is allowed in C99 and explicitly allowed in C11. union { Msg msg; unsigned int asBuffer[sizeof(Msg)/sizeof(unsigned int)]; }; You can disable strict aliasing in your compiler (f[no-]strict-aliasing in gcc)) You can use char* for aliasing instead of your system's word. The rules allow an exception for char* (including signed char and unsigned char). It's always assumed that char* aliases other types. However this won't work the other way: there's no assumption that your struct aliases a buffer of chars.

初学者要小心

这只是将两种类型叠加在一起时的一个潜在雷区。您还应该了解字节顺序、单词对齐，以及如何通过正确打包结构来处理对齐问题。

脚注

C 2011 6.5 7允许左值访问的类型有:

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.

作为Doug T.已经写过的附录，在这里 是一个简单的测试用例，可能会触发GCC:

check.c

#include <stdio.h>

void check(short *h,long *k)
{
    *h=5;
    *k=6;
    if (*h == 5)
        printf("strict aliasing problem\n");
}

int main(void)
{
    long      k[1];
    check((short *)k,k);
    return 0;
}

编译gcc -O2 -o check check.c。 通常(我尝试过的大多数gcc版本)这会输出“严格的混叠问题”，因为编译器假设“h”不能与“check”函数中的“k”地址相同。因此，编译器会优化if (*h == 5)，并始终调用printf。

对于那些感兴趣的人，这里有x64汇编代码，由gcc 4.6.3生成，运行在ubuntu 12.04.2的x64上:

movw    $5, (%rdi)
movq    $6, (%rsi)
movl    $.LC0, %edi
jmp puts

所以if条件从汇编代码中完全消失了。