Visual C++没事别启用/Za编译选项

Visual C++对于C++标准的支持不是很完善好像是钦定了的。MS还特意在这里说了些非标准行为[1]以及扩展行为[2]。这就不可避免地会让处女座程序猿感到难受（我不是处女座）。所以，经过一番google/bing之后，你会找到一个很好的VC++编译选项/Za。

我们先来看看/Za选项是什么意思：

The /Za compiler option emits an error for language constructs that are not compatible with ANSI C89 or ISO C++11.

We recommend that you disable language extensions by specifying the /Za option if you plan to port your program to other environments. When /Za is specified, the compiler treats Microsoft extended keywords as simple identifiers, disables the other Microsoft extensions, and automatically defines the __STDC__ predefined macro for C programs.

说得和真的一样，让人非常激动，一颗赛艇。

当然了，事实并非你想得那么完美[手动滑稽.gif]。

你include <windows.h>试一下[蜜汁微笑.avi]。

你要还不死心，看这里。

When I encountered that bug (valid code like vector<unique_ptr> triggers compiler errors under /Za; the problem is that /Za performs an elided-copy-constructor accessibility check during move construction when no copy constructor is being called even theoretically), I asked for a fix, but unfortunately the resolution was "won't fix forever". I can confirm that it will not be fixed in VC10 SP1 RTW, and that there are currently no plans to fix it in VC11.

As a result, during VC10's development we stopped testing our C++ Standard Library implementation under /Za, and I no longer consider it to be a supported switch. (We support almost all switches, including the headache-inducing ones like /Gr and /Gz, but we have to draw the line when valid code is broken.)

I also asked for /Za to be removed from VC11, but that won't happen either. (However, I was told that /Wp64, which was deprecated in VC9 and VC10, will be removed from VC11. I don't think that's happened yet so I'll have to ping a compiler dev about it. /Wp64 has always been broken - it suffers from false positives and false negatives and interacts extremely badly with templates - and it has been completely superseded by the existence of x64-targeting compilers.)

补充说明：

【2017/8/10】正确的姿势应该是使用/Zc。

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[1] Visual C++ 非标行为：https://docs.microsoft.com/en-us/cpp/cpp/nonstandard-behavior

[2] Visual C++扩展行为：https://docs.microsoft.com/en-us/cpp/build/reference/microsoft-extensions-to-c-and-cpp