如何使C ++编译器使用无符号到有符号的技巧优化比较 [英] How to make the C++ compiler to use the unsigned to signed trick for optimization of comparison

问题描述

所以从我知道以下两个函数的行为应该是完全一样的。
然而，如果我实现使用第一个，编译器似乎不能意识到它相当于第二个。是否还有提示编译器使用第二个？

So from what I know the following two functions' behavior should be exactly the same. However, if I implement using the first one, the compiler seems can't realize it's equivalent to the second. Is there anyway to hint the compiler to use the second ?

换句话说，我讨厌做static_cast的东西...反正要避免它？

In another words, I hate to do the static_cast things... Anyway to avoid it?

// In my real use case, range will never be negative....
// All I want is to return (diff > range || diff < 0) assuming range >=0
bool IsOutofRange1(int diff, int range) {
  if ((range >= 0) && (diff > range || diff < 0)) return true;
  if (range < 0 && diff > range && diff < 0) return true;
  return false;
}

bool IsOutofRange2(int diff, int range) {
  if (static_cast<unsigned>(diff) > static_cast<unsigned>(range)) return true;
  return false;
}

推荐答案

，为什么你使用一个签名的类型（是的，我知道你讨厌铸造）？如何重载：

If your range in never negative, why do you use a signed type for it (yeah, I know you hate casting) ? How about overloading:

bool IsOutofRange(int diff, int range)
{
    ....
}

bool IsOutofRange(int diff, unsigned range)
{
    if (static_cast<unsigned>(diff) > range) return true;
    return false;
}

当然强制编译器使用第二个版本你会...）：

Of course to force compiler to use the second version you would (oh my God...):

IsOutofRange(diff,static_cast<unsigned>(range));

我希望对我有帮助，对static_cast会感到非常抱歉。

I hope that helps, and I am deeply sorry about the static_cast'ing.

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