我试图编写一个函数以生成任意嵌套的 vector ,并使用C++中的给定特定值进行初始化。例如,auto test_vector = n_dim_vector_generator<2, long double>(static_cast<long double>(1), 1);
有望创建一个类型为std::vector<std::vector<long double>>
的“test_vector”对象。该test_vector的内容应与以下代码相同。
std::vector<long double> vector1;
vector1.push_back(1);
std::vector<std::vector<long double>> test_vector;
test_vector.push_back(vector1);
n_dim_vector_generator函数的更复杂用法:
auto test_vector2 = n_dim_vector_generator<15, long double>(static_cast<long double>(2), 3);
在这种情况下,参数
static_cast<long double>(2)
作为 vector 中的数据,数字3
作为推送时间。因此,此test_vector2的内容应与以下代码相同。 std::vector<long double> vector1;
vector1.push_back(static_cast<long double>(2));
vector1.push_back(static_cast<long double>(2));
vector1.push_back(static_cast<long double>(2));
std::vector<std::vector<long double>> vector2;
vector2.push_back(vector1);
vector2.push_back(vector1);
vector2.push_back(vector1);
std::vector<std::vector<std::vector<long double>>> vector3;
vector3.push_back(vector2);
vector3.push_back(vector2);
vector3.push_back(vector2);
//...Totally repeat 15 times in order to create test_vector2
std::vector<...std::vector<long double>> test_vector2;
test_vector2.push_back(vector14);
test_vector2.push_back(vector14);
test_vector2.push_back(vector14);
实现n_dim_vector_generator函数的细节如下。
#include <iostream>
#include <vector>
template <typename T, std::size_t N>
struct n_dim_vector_type;
template <typename T>
struct n_dim_vector_type<T, 0> {
using type = T;
};
template <typename T, std::size_t N>
struct n_dim_vector_type {
using type = std::vector<typename n_dim_vector_type<T, N - 1>::type>;
};
template<std::size_t N, typename T>
typename n_dim_vector_type<T,N>::type n_dim_vector_generator(T t, unsigned int);
template <std::size_t N, typename T>
typename n_dim_vector_type<T, N>::type n_dim_vector_generator<N, T>(T input_data, unsigned int push_back_times) {
if (N == 0)
{
return std::move(input_data);
}
typename n_dim_vector_type<T, N>::type return_data;
for (size_t loop_number = 0; loop_number < push_back_times; loop_number++)
{
return_data.push_back(n_dim_vector_generator<N - 1, T>(input_data, push_back_times));
}
return return_data;
}
结果,我收到了一个
'return': cannot convert from 'long double' to 'std::vector<std::vector<long double,std::allocator<long double>>,std::allocator<std::vector<long double,std::allocator<long double>>>>'
错误,我知道这是由if (N == 0)
块引起的,这是递归结构的终止条件。但是,如果我尝试将终止条件编辑为单独的形式。template <typename T>
inline T n_dim_vector_generator<0, T>(T input_data, unsigned int push_back_times) {
return std::move(input_data);
}
template <std::size_t N, typename T>
typename n_dim_vector_type<T, N>::type n_dim_vector_generator<N, T>(T input_data, unsigned int push_back_times) {
typename n_dim_vector_type<T, N>::type return_data;
for (size_t loop_number = 0; loop_number < push_back_times; loop_number++)
{
return_data.push_back(n_dim_vector_generator<N - 1, T>(input_data, push_back_times));
}
return return_data;
}
错误
'n_dim_vector_generator': illegal use of explicit template arguments
发生了。有没有更好的解决方案来解决这个问题?开发环境在Windows 10 1909中,带有Microsoft Visual Studio Enterprise 2019版本16.4.3
最佳答案
要实现您的特定映射:
auto test_vector = n_dim_vector_generator<2, long double>(2, 3)
填充为2的3x3 vector ,如果您利用此
vector
构造函数,则模板可能会更简单:std::vector<std::vector<T>>(COUNT, std::vector<T>(...))
由于
vector
是可复制的,因此它将用该 vector 的其他副本填充COUNT个插槽。所以...template <size_t N, typename T>
struct n_dim_vector_generator {
using type = std::vector<typename n_dim_vector_generator<N-1, T>::type>;
type operator()(T value, size_t size) {
return type(size, n_dim_vector_generator<N-1, T>{}(value, size));
}
};
template <typename T>
struct n_dim_vector_generator<0, T> {
using type = T;
type operator()(T value, size_t size) {
return value;
}
};
用法:
auto test_vector = n_dim_vector_generator<2, long double>{}(2, 3);
演示:https://godbolt.org/z/eiDAUG
为了记录起见,为了解决注释中的一些问题,C++具有与多维C数组等效的惯用,可初始化,连续内存类:嵌套
std::array
:std::array<std::array<long double, COLUMNS>, ROWS> test_array = { /*...*/ };
for (auto& row : test_array)
for (auto cell : row)
std::cout << cell << std::endl;
如果要减少样板声明一个,可以使用一个结构:
template <typename T, size_t... N>
struct multi_array;
template <typename T, size_t NFirst, size_t... N>
struct multi_array<T, NFirst, N...> {
using type = std::array<typename multi_array<T, N...>::type, NFirst>;
};
template <typename T, size_t NLast>
struct multi_array<T, NLast> {
using type = std::array<T, NLast>;
};
template <typename T, size_t... N>
using multi_array_t = typename multi_array<T, N...>::type;
然后使用:
multi_array_t<long double, ROWS, COLUMNS> test_array = { /*...*/ };
for (auto& row : test_array)
for (auto cell : row)
std::cout << cell << std::endl;
它像C数组一样分配在堆栈上。当然,这会吞噬您的堆栈空间。但是您可以在
std::unique_ptr
周围设置装饰器范围,以使指向该指针的指针更易于访问:template <typename T, size_t... N>
struct dynamic_multi_array : std::unique_ptr<multi_array_t<T, N...>> {
using std::unique_ptr<multi_array_t<T, N...>>::unique_ptr;
constexpr typename multi_array_t<T, N...>::value_type& operator [](size_t index) { return (**this)[index]; }
constexpr const typename multi_array_t<T, N...>::value_type& operator [](size_t index) const { return (**this)[index]; }
constexpr typename multi_array_t<T, N...>::iterator begin() { return (**this).begin(); }
constexpr typename multi_array_t<T, N...>::iterator end() { return (**this).end(); }
constexpr typename multi_array_t<T, N...>::const_iterator begin() const { return (**this).begin(); }
constexpr typename multi_array_t<T, N...>::const_iterator end() const { return (**this).end(); }
constexpr typename multi_array_t<T, N...>::const_iterator cbegin() const { return (**this).cbegin(); }
constexpr typename multi_array_t<T, N...>::const_iterator cend() const { return (**this).cend(); }
constexpr typename multi_array_t<T, N...>::size_type size() const { return (**this).size(); }
constexpr bool empty() const { return (**this).empty(); }
constexpr typename multi_array_t<T, N...>::value_type* data() { return (**this).data(); }
constexpr const typename multi_array_t<T, N...>::value_type* data() const { return (**this).data(); }
};
(如果您将这些方法与
nullptr
一起使用,请买家当心)然后,您仍然可以对
new
表达式进行初始化,并将其像容器一样使用:dynamic_multi_array<long double, ROWS, COLUMNS> test_array {
new multi_array_t<long double, ROWS, COLUMNS> { /* ... */ }
};
for (auto& row : test_array)
for (auto cell : row)
std::cout << cell << std::endl;
演示:https://godbolt.org/z/lUwVE_
关于c++ - 在C++中生成任意嵌套的 vector ,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/59970668/