Narrowing conversions and initializer lists, which

问题:

Considering the following piece of code:

#include <iostream>

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

  return 0;
}

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• When compiled in GCC4.9 it compiles fine with only a warning:

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

• Compiling with either Clang3.3 or VC++2013 gives a compile error:

  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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Questions:

• Which of the compilers is right according to the standard?

• Is there any reason why the compilers mentioned above should exhibit such diverse behaviour?

回答1:

The answer

Both compilers are correct!

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Explanation

The Standard doesn't distinguish between an error and a warning, both go under the category of Diagnostics.

1.3.6 diagnostic message [defns.diagnostic]

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

Since the Standard says that a diagnostic is required in case a program is ill-formed, such as when a narrowing-conversion takes place inside a braced-initializer, both compilers are confirming.

Even if the program is ill-formed from the Standards point of view, it doesn't mandate that a compiler halts compilation because of that; an implementation is free to do whatever it wants, as long as it issues a diagnostic.

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The reason for gcc's behavior?

Helpful information was provided by @Jonathan Wakely through comments on this post, below are a merge of the two comments;

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.