std :: tuple参数中的类型顺序有什么影响吗？ [英] Does type order in std::tuple arguments have any effects?

问题描述

假设我想在一个元组中存储三种类型： int ， float 和 std :: vector< double>

Say I want to store three types in a tuple : int, float and std::vector<double>

，

tuple<int, float, vector<int>> t;

与此之间有任何差异

tuple<vector<int>, int, float> t;

由于执行 tuple 类变量基数，我预期一个不同的布局为生产的类，但它以任何方式重要吗？在 tuple 中放置类型时（例如放最大的第一个等等），还有什么需要考虑的优化注意事项？

Due to the implementation of tuple as a class of variadic bases, I'm expecting a different layout for the produced classes, but does it matter in any way ? Also are there any optimization considerations to take into account, when placing types in a tuple (eg put the largest first etc) ?

推荐答案

标准对类型的实际布局没有任何限制。该顺序影响的唯一的事情是 std :: get< N> ， std :: tuple_element< N，T> 等等。

The standard doesn't place any restrictions on the actual layout of the types. The only things the order influences are the results of std::get<N>, std::tuple_element<N, T> and so on.

我知道libstdc ++和Visual C ++按照给定顺序的相反顺序排列类型; libc ++按给定顺序排列类型。这实质上意味着没有可移植的方式来选择总是产生最佳布局的订单。

I know that libstdc++ and Visual C++ lay out the types in reverse order of the order given; libc++ lays out the types in the order given. This essentially means that there is no portable way to pick an order that always produces the best layout.

但是其他订单也是可能的。允许实施元组，使用总是产生最小尺寸的布局，但是仍保留 std :: get< N> 的相同语义，等等。我不知道这样做的任何标准库实现。

Other orders are possible, though. An implementation is allowed to implement tuple with a layout that always produces minimal size but still preserves the same semantics for std::get<N> and so on. I don't know of any standard library implementation that does this, though.

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