performing `a = b` is equivalent to `a = std::move(b)`. No copy construction or copy assignment allowed.
performing `a = b` is equivalent to `a = std::move(b)`. No copy construction or copy assignment allowed.执行 `a = b` 等同于 `a = std::move(b)`,且不允许复制构造和复制赋值。
performing `a = b` is equivalent to `a = std::move(b)`. No copy construction or copy assignment allowed.执行 `a = b` 等同于 `a = std::move(b)`,且不允许复制构造和复制赋值。