如何打印 n 维 C++ STL 容器？容器是数组的数组或向量的向量

Question

我有一个代码，我想在其中显示表示为向量的向量或 std::arrays of std::arrays 的张量。目的是按照 numpy 打印它们的方式打印它们。我仍在学习 C++ 中的元编程，并想探索如何使用函数模板打印 n-dim 容器，该函数模板可以接受这个容器容器并递归迭代它，return 一个我稍后可以计算的字符串。

Numpy 示例：

>>> np.ones([2,2])
array([[1., 1.],
       [1., 1.]])
>>> np.ones([2,2,4])
array([[[1., 1., 1., 1.],
        [1., 1., 1., 1.]],

       [[1., 1., 1., 1.],
        [1., 1., 1., 1.]]])
>>> np.ones(4)
array([1., 1., 1., 1.])
>>>

目前已尝试：我尝试了此处接受的响应作为响应：

Print simply STL vectors of vectors recursively in C++

它确实适用于 2 个暗向量，但是当我在 printContainerV2() 中将对 printContainer() 的调用更改为 printContainerV2() 时，使用 3d 向量编译失败：

代码:

#include <iostream>
#include <iterator>
#include <vector>

template <typename Iter, typename Cont>
bool isLast(Iter iter, const Cont& cont)
{
    return (iter != cont.end()) && (next(iter) == cont.end());
}


template <typename T>
struct is_cont {
    static const bool value = false;
};

template <typename T,typename Alloc>
struct is_cont<std::vector<T,Alloc> > {
    static const bool value = true;
};


template <typename T>
std::string printContainer(T const& container)
{
    std::string str = "{";
    for (auto it = std::begin(container); it != std::end(container); ++ it)
        if (isLast(it, container))
                str = str + std::to_string(*it) + "}";
        else
                str = str + std::to_string(*it) + ",";
    return str;
}

template<typename T>
using if_not_cont = std::enable_if<!is_cont<T>::value, T>;

template<typename T>
using if_cont = std::enable_if<is_cont<T>::value, T>;

template <typename T, typename std::enable_if<!is_cont<T>::value, T>::type* = nullptr>
std::string printContainerV2(T const& container)
{
    std::string str = "{";
    for (auto it = std::begin(container); it != std::end(container); ++ it)
        if (isLast(it, container))
                str = str + std::to_string(*it) + "}";
        else
                str = str + std::to_string(*it) + ",";
    return str;
}

template <typename T, typename std::enable_if<is_cont<T>::value, T>::type* = nullptr>
std::string printContainerV2(T const& container)
{
    std::string str = "{";
    for (auto it = std::begin(container); it != std::end(container); ++ it)
        if (isLast(it, container))
                str = str + printContainerV2(*it) + "}";
        else
                str = str + printContainerV2(*it) + ",";
    return str;
}

int main()
{
    std::vector<int> A({2,3,6,8});
    std::vector<std::vector<int>> M(2,A);
    std::vector<std::vector<std::vector<float>>> m3{{{1,2}, {3,4}},{{5,6}, {7,8}},{{1,2}, {5,9}}};

    std::cout << is_cont<decltype(A)>::value << std::endl;  // returns true !

    // for (auto it = std::begin(M); it != std::end(M); ++ it)
    // {
    //     std::cout << printContainer(*it) << std::endl; // works well std::vector<int>
    //     std::cout << is_cont<decltype(*it)>::value << std::endl; // return false :(
    // }

    // Want to use this for printing a std::vector<std::vector<int>>
    std::cout << printContainerV2(M) << std::endl; // not working !
    std::cout << printContainerV2(m3) << std::endl; // not working
}

命令：clang++ --std=c++17 test.cpp test.cpp是上面代码的名字。

我收到这个错误:

test.cpp:45:20: error: no matching function for call to 'begin'
    for (auto it = std::begin(container); it != std::end(container); ++ it)
                   ^~~~~~~~~~
test.cpp:59:29: note: in instantiation of function template specialization 'printContainerV2<int, nullptr>'
      requested here
                str = str + printContainerV2(*it) + "}";
                            ^
test.cpp:59:29: note: in instantiation of function template specialization 'printContainerV2<std::__1::vector<int,
      std::__1::allocator<int> >, nullptr>' requested here
test.cpp:80:19: note: in instantiation of function template specialization
      'printContainerV2<std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >,
      std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >, nullptr>' requested here
     std::cout << printContainerV2(M) << std::endl; // not working !
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/initializer_list:99:1: note: 
      candidate template ignored: could not match 'initializer_list<type-parameter-0-0>' against 'int'
begin(initializer_list<_Ep> __il) _NOEXCEPT
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iterator:1753:1: note: 
      candidate template ignored: could not match '_Tp [_Np]' against 'const int'
begin(_Tp (&__array)[_Np])
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iterator:1771:1: note: 
      candidate template ignored: substitution failure [with _Cp = const int]: member reference base type
      'const int' is not a structure or union
begin(_Cp& __c) -> decltype(__c.begin())
^                              ~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iterator:1779:1: note: 
      candidate template ignored: substitution failure [with _Cp = int]: member reference base type 'const int' is
      not a structure or union
begin(const _Cp& __c) -> decltype(__c.begin())
^                                    ~
1 error generated.

Answer 1

这是打印任意维度向量的简单方法：

template<typename T>
std::ostream& operator<<(std::ostream& out, const std::vector<T> &vec){
    out << "[ ";
    for(const auto& t: vec){
        out << t << " ";
    }
    out << "] ";
    return out;
}

对于任何其他容器，您可以执行完全相同的操作。

使用：

int main()
{
    std::vector<int> v{1,2,3};
    std::cout << v << std::endl;
    std::vector<std::vector<std::vector<int>>> v_ds
        {
            {{{1,2,3},{4,5,6}},{{7,8},{9,10}}, {{1,2,3},{4,5,6}},{{7,8},{9,10}}}
        };
    std::cout << v_ds << std::endl;
    return 0;
}

编辑： 这是 operator<< 的一个版本，它可以打印任何可以迭代的内容（例如 std::array、std::vector、std::list，甚至是已知边界的 c 数组）：

template<typename Container, typename = 
    std::enable_if_t<std::is_same_v<std::void_t<
        decltype(static_cast<typename Container::const_iterator (*)(const Container&)>(&std::cbegin)),
        decltype(static_cast<typename Container::const_iterator (*)(const Container&)>(&std::cend))>, void>
        && !std::is_same_v<std::string, Container>>>
std::ostream& operator<<(std::ostream& out, const Container &vec)
{
    out << "[ ";
    for(const auto& t: vec){
        out << t << " ";
    }
    out << "] ";
    return out;
}

乱七八糟的，我知道，但是这样的语法糖有用:)

另一项编辑： 我之前发布的版本在使用 c 数组时略有错误。这是一个更新版本：

template<class...> constexpr bool true_t = true; // replacement of void_t

template<typename Container>
auto operator<<(std::ostream& out, Container&& vec) -> 
    std::enable_if_t<
        true_t<
            decltype(std::cbegin(vec)),
            decltype(std::cend(vec))
        >
        && !std::is_convertible_v<Container, std::string_view>,
        std::ostream&
    >
{
    out << "[ ";
    for(const auto& t: vec){
        out << t << " ";
    }
    out << "] ";
    return out;
}

Answer 2

当你的第一个printContainerV2启用时，!is_cont<T>::value是true，这意味着此时你的T不再是container类型.在你的例子中，它们是 int，所以你不能在 int 上调用 std::begin，你应该直接 return std::to_string(value).

template <typename T, typename std::enable_if<!is_cont<T>::value, T>::type* = nullptr>
std::string printContainerV2(T const& value)
{
    return std::to_string(value);
}

Demo.

Answer 3

对于 C++17 中的嵌套容器，区分类型，你可以这样做：

#include <bits/stdc++.h>
using namespace std;

template <typename, typename = void> constexpr bool is_iterable{};
template <> constexpr bool is_iterable<string> = false;
template <typename T, size_t N> constexpr bool is_iterable<T[N]> = true;
template <typename T> constexpr bool is_iterable<T, void_t<decltype(declval<T>().begin()), decltype(declval<T>().end())>> = true;

template <typename> constexpr bool is_set{};
template <typename T> constexpr bool is_set<set<T>> = true;
template <typename T, typename U> constexpr bool is_set<map<T, U>> = true;

template <typename> constexpr bool is_tuple{};
template <typename... T> constexpr bool is_tuple<tuple<T...>> = true;
template <typename T, typename U> constexpr bool is_tuple<pair<T, U>> = true;

inline string to_string(string s) { return '"' + s + '"'; }
inline string to_string(char c) { return '\'' + string(1, c) + '\''; }

template<typename T>
string print(const T& t, string&& s = string(), size_t depth = 0) {
    constexpr size_t TAB_SPACES = 2;
    if constexpr (!is_tuple<T> and !is_iterable<T>) {
        if (s.back() == '\n') s.pop_back();
        s += to_string(t) + ", ";
    } else {
        pair<string, string> braces = is_tuple<T> ? pair("(", ")") : is_set<T> ? pair("{", "}") : pair("[", "]");
        s += string(TAB_SPACES * depth, ' ') + braces.first + '\n';
        if constexpr (is_tuple<T>)
            if constexpr (tuple_size_v<T> == 0) s.push_back('[=10=]');
            else apply([&s, depth](const auto&... x){ ((s = print(x, move(s), depth + 1)), ...); }, t);
        else if (begin(t) == end(t)) s.push_back('[=10=]');
        else for_each(begin(t), end(t), [&s, depth](const auto& x){ s = print(x, move(s), depth + 1); });
        s.erase(s.length() - 2);
        if (s.back() == ')' or s.back() == '}' or s.back() == ']') s += '\n' + string(TAB_SPACES * depth, ' ');
        s += braces.second + ",\n";
    }
    if (depth == 0) cout << s.erase(s.length() - 2) << endl;
    return move(s);
}

然后：

vector<map<int, string>> ex = {{{1, "a"}, {2, "b"}}};
print(ex);

将打印：

[
  {
    (1, "a"),
    (2, "b")
  }
]

用于脚本模板，其中优先考虑简单性和无外部依赖。生产可能由图书馆更好地服务。

如何打印 n 维 C++ STL 容器？容器是数组的数组或向量的向量

How to print a n-dimensional c++ STL container? Container is array of arrays or vectors of vectors

c++

c++14

c++17