为什么 C++11 不允许使用自动进行直接列表初始化
Why doesn't c++11 allow direct-list-initialization with auto
我朋友告诉我的
auto x1 = {3}; // x1 is std::initializer_list<int>
auto x2{1, 2}; // error: not a single element
auto x3{3}; // x3 is int
不太明白为什么auto x2{1, 2};不合法，难道不能直接推导为std::initializer_list<int>吗？
根据我的编译器，这是因为 x2 是一个标量，因此只能有一个索引。然而，下面的编译是因为它是一个数组并且数组有两个索引。
自动 x2[]{1,2};
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.
有关详细信息，请参阅 ISO/IEC JTC1/SC22/WG21 C++ 标准委员会关于 Auto and braced-init-lists 的文件。
我朋友告诉我的
auto x1 = {3}; // x1 is std::initializer_list<int>
auto x2{1, 2}; // error: not a single element
auto x3{3}; // x3 is int
不太明白为什么auto x2{1, 2};不合法，难道不能直接推导为std::initializer_list<int>吗？
根据我的编译器，这是因为 x2 是一个标量，因此只能有一个索引。然而，下面的编译是因为它是一个数组并且数组有两个索引。
自动 x2[]{1,2};
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.
有关详细信息，请参阅 ISO/IEC JTC1/SC22/WG21 C++ 标准委员会关于 Auto and braced-init-lists 的文件。