c++ - 缩小转换和初始化列表，哪个编译器是对的？

Question

考虑以下代码:

#include <iostream>

auto main() -> int {
  double x(7.0);
  int i{x};
  std::cout << "i = " << x << std::endl;

  return 0;
}

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• 在 GCC4.9 中编译时，它编译得很好，只有一个警告:

  warning: narrowing conversion of ‘x’ from ‘double’ to ‘int’ inside { }

• 使用 Clang3.3 或 VC++2013 编译会出现编译错误:

  error: type 'double' cannot be narrowed to 'int' in initializer list error C2397: conversion from 'double' to 'int' requires a narrowing

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问题:

• 根据标准，哪个编译器是正确的？

• 上面提到的编译器有什么理由表现出如此多样化的行为吗？

Answer 1

答案

两个编译器都是正确的!

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说明

标准不区分错误和警告，两者都属于诊断。

1.3.6 diagnostic message [defns.diagnostic]

message belonging to an implementation-defined subset of the implementation's output messages

由于标准规定在程序格式错误的情况下需要进行诊断，例如在大括号初始化器内发生缩小转换时，两个编译器都在确认。

即使从标准的角度来看程序格式不正确，编译器也不会因此而停止编译；一个实现可以自由地做它想做的任何事情，只要它发出一个诊断。

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gcc 行为的原因？

@Jonathan Wakely 提供了有用的信息通过对这篇文章的评论，以下是两条评论的合并；

he exact reason is that GCC made it an error at one point and it broke ALL THE PROGRAMS so it got turned into a warning instead. Several people who turned on the -std=c++0x option for large C++03 codebases found harmless narrowing conversions to cause most of the porting work to go to C++11

See e.g. PR 50810 where Alisdair reports narrowing errors were >95% of the problems in Bloomberg's code base.

In that same PR you can see that unfortunately it wasn't a case of "let's just issue a warning and be done with it" because it took a lot of fiddling to get the right behaviour.