除了Matthieu的答案之外,您还可以自动计算可迭代/容器的维度数量。但首先,我们必须设定一个规则,即何时某物是可迭代/容器:
template<class T, class R = void>
struct AliasWrapper {
using Type = R;
};
template<class T, class Enable = void>
struct HasValueType : std::false_type {};
template<class T>
struct HasValueType<T, typename AliasWrapper<typename T::value_type>::Type> : std::true_type {};
template<class T, class Enable = void>
struct HasConstIterator : std::false_type {};
template<class T>
struct HasConstIterator<T, typename AliasWrapper<typename T::const_iterator>::Type> : std::true_type {};
template<class T, class Enable = void>
struct HasIterator : std::false_type {};
template<class T>
struct HasIterator<T, typename AliasWrapper<typename T::iterator>::Type> : std::true_type {};
template<class T>
struct IsIterable {
static constexpr bool value = HasValueType<T>::value && HasConstIterator<T>::value && HasIterator<T>::value;
};
我们可以按如下方式计算维度:
template<class T, bool IsCont>
struct CountDimsHelper;
template<class T>
struct CountDimsHelper<T, true> {
using Inner = typename std::decay_t<T>::value_type;
static constexpr int value = 1 + CountDimsHelper<Inner, IsIterable<Inner>::value>::value;
};
template<class T>
struct CountDimsHelper<T, false> {
static constexpr int value = 0;
};
template<class T>
struct CountDims {
using Decayed = std::decay_t<T>;
static constexpr int value = CountDimsHelper<Decayed, IsIterable<Decayed>::value>::value;
};
我们可以创建一个视图包装器,其中包含一个
begin()
和
end()
函数。
template<class Iterable, int Dims>
class Flatten {
public:
using iterator = FlattenIterator<Iterable, Dims>;
private:
iterator _begin{};
iterator _end{};
public:
Flatten() = default;
template<class I>
explicit Flatten(I&& iterable) :
_begin(iterable),
_end(iterable)
{}
iterator begin() const {
return _begin;
}
iterator end() const {
return _end;
}
};
为了让对象
Flatten
的创建更加容易,我们定义了一个辅助函数:
template<class Iterable>
Flatten<std::decay_t<Iterable>, CountDims<Iterable>::value - 1> flatten(Iterable&& iterable) {
return Flatten<std::decay_t<Iterable>, CountDims<Iterable>::value - 1>(iterable);
}
使用方法:
std::vector<std::vector<int>> vecs = {{1,2,3}, {}, {4,5,6}};
for (int i : flatten(vecs)) {
}