调用约定和评估秩序 [英] calling convention and evaluation order

问题描述

我知道，C ++不指定哪些参数传递给函数的顺序。但是，如果我们编写以下code：

 无效__cdecl FUNC（INT A，INT B，INT C）
{
       的printf（％D，％D，％D，A，B，C）;
}
诠释的main（）
{
   INT I = 10;
   FUNC（++我，我，我++）;
}
 

我们可以肯定地说，输出将 12,11,11 自的 __ CDECL 确保参数传递顺序是从右到左？

解决方案

按照标准，有两件事情你需要理解和区分：

  

  
1. C ++不指定哪些参数的通过以一个订单
     功能（像你说的你自己，
     是真的！）

  
2. C ++不指定该函数的参数是顺序
     评估 [expr.call]。

  

现在，请注意， __ CDECL 既保证了第一，没有第二个。 调用约定决定的仅如何在函数的参数能否通过，左到右或从右至左; 他们仍然可以以任意顺序评价过！

希望这澄清了有关调用约定你的疑惑。

然而，由于这些公约都是微软编译器扩展到C ++，所以你的code是不可移植。在这种情况下，你可以看到MSVC ++编译器如何评估函数的参数，并放宽如果你不想上运行其他平台一样code！

---

  FUNC（++我，我，我++）;
 

请注意，这个特殊的code调用不确定的行为，因为 I 递增一次以上的没有的任何干预的任何序列点。

I know that C++ doesn't specify the order in which parameters are passed to a function. But if we write the following code:

void __cdecl func(int a, int b, int c)
{
       printf("%d,%d,%d", a,b,c);
}
int main()
{
   int i=10;
   func(++i, i, ++i);
}

Can we reliably say the output would be 12,11,11 since the __cdecl ensures that argument-passing order is right to left?

解决方案

As per the Standard, there are two things you need to understand and differentiate:

1. C++ doesn't specify the order in which parameters are passed to a function (as you said yourself, that is true!)
2. C++ doesn't specify the order in which the function arguments are evaluated [expr.call].

Now, please note, __cdecl ensures only the first, not the second. Calling conventions decide only how the functions arguments will be passed, left-to-right or right-to-left; they can still be evaluated in ANY order!

Hope this clarifies your doubts regarding the calling conventions.

However, since these conventions are Microsoft compiler extension to C++, so your code is non-portable. In that case, you can see how MSVC++ compiler evaluates function arguments and be relax IF you don't want to run the same code on other platform!

---

func(++i, i, ++i);

Note that this particular code invokes undefined behavior, because i is incremented more than once without any intervening any sequence point.

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