c++ - 为什么C++ 11不允许使用auto进行直接列表初始化

Question

我的 friend 告诉我

auto x1 = {3}; // x1 is std::initializer_list<int>
auto x2{1, 2}; // error: not a single element
auto x3{3};    // x3 is int

我不太了解 auto x2{1, 2};为什么是非法的，难道不能直接推断为 std::initializer_list<int>吗？

Answer 1

该标准有意限制了这种情况。

auto x2{1, 2}; // error: not a single element

Auto and braced initializers cause a teachability problem; we want to teach people to use uniform initialization, but we need to specifically tell programmers to avoid braces with auto. In C++14, we now have more cases where auto and braces are problematic; return type deduction for functions partially avoids the problem, since returning a braced-list won't work as it's not an expression. However, returning an auto variable initialized from a braced initializer still returns an initializer_list, inviting undefined behaviour. Lambda init captures have the same problem. This paper proposes to change a brace-initialized auto to not deduce to an initializer list, and to ban brace-initialized auto for cases where the braced-initializer has more than one element.

有关更多详细信息，请参见关于 Auto and braced-init-lists的ISO / IEC JTC1 / SC22 / WG21 C++标准委员会的文件。