在 C++ 中转换为 bool 对性能有什么影响?

问题描述

[这个问题与这个.]

我的编译器警告将某些类型隐式转换或强制转换为 bool 而显式转换不会产生警告:

My compiler warns about implicitly converting or casting certain types to bool whereas explicit conversions do not produce a warning:

long t = 0;
bool b = false;
b = t;                    // performance warning: forcing long to bool
b = (bool)t;              // performance warning
b = bool(t);              // performance warning
b = static_cast<bool>(t); // performance warning
b = t ? true : false;     // ok, no warning
b = t != 0;               // ok
b = !!t;                  // ok

这是使用 Visual C++ 2008 但我怀疑其他编译器可能有类似的警告.

This is with Visual C++ 2008 but I suspect other compilers may have similar warnings.

所以我的问题是:强制转换/转换为 bool 的性能影响是什么?在某些情况下(例如，对于某些目标架构或处理器)，显式转换是否具有更好的性能?隐式转换是否会以某种方式混淆优化器?

So my question is: what is the performance implication of casting/converting to bool? Does explicit conversion have better performance in some circumstance (e.g., for certain target architectures or processors)? Does implicit conversion somehow confuse the optimizer?

Microsoft 对其警告的解释并不是特别有用.他们暗示有一个很好的理由，但他们没有解释.

Microsoft's explanation of their warning is not particularly helpful. They imply that there is a good reason but they don't explain it.

推荐答案

我对这种行为感到困惑，直到我找到了这个链接:

I was puzzled by this behaviour, until I found this link:

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=99633

显然，来自拥有"此警告的 Microsoft 开发人员:

Apparently, coming from the Microsoft Developer who "owns" this warning:

这个警告令人惊讶有帮助，并在我的代码中发现了一个错误就在昨天.我认为马丁是去掉性能警告"上下文.