具有矢量的 C++ 显示地图

Question

std::map<std::string, std::vector<string>> data;

为了使用copy 打印出来，我的std::ostream_iterator 应该是什么？

显然std::ostream_iterator<std::pair<std::string, std::vector<std::string>>> out_it(std::cout, "\n"); 没有成功。

我的operator<< 重载是以下std::ostream& operator<<(std::ostream& out, const std::pair<std::string, std::vector<std::string>>& p)，它写出p.first 和p.second 并返回它。

Answer 1

如果您使用 C++ 进行任何认真的编程，您最终将需要一种通用的方式来打印出集合。

这是一个基础：

#include <iostream>
#include <map>
#include <vector>
#include <string>


// introduce the concept of an object that emits values to an ostream
// by default it simply calls operator <<
template<class T> struct emitter
{
    using arg_type = T;

    emitter(const T& v) : v_(v) {}

    friend std::ostream& operator<<(std::ostream& os, const emitter& e) {
        return os << e.v_;
    }

    const T& v_;
};

// introduce the concept of an io manipulator called emit
template<class T> auto emit(const T& v) -> emitter<T>
{
    return emitter<std::decay_t<T>>(v);
}

// specialise the emitter for maps
template<class K, class V, class C, class A>
struct emitter<std::map<K, V, C, A>>
{
    using arg_type = std::map<K, V, C, A>;

    emitter(const arg_type& v) : v_(v) {}

    friend std::ostream& operator<<(std::ostream& os, const emitter& e) {
        const char* elem_sep = "\n\t";
        const char* end_sep = " ";
        os << "{";
        for (const auto& elem : e.v_)
        {
            os << elem_sep << emit(elem.first) << ": " << emit(elem.second);
            end_sep = "\n";
        }

        return os << end_sep << "}";
    }

    const arg_type& v_;
};

// specialise the emitter for vectors
template<class V, class A>
struct emitter<std::vector<V, A>>
{
    using arg_type = std::vector<V, A>;

    emitter(const arg_type& v) : v_(v) {}

    friend std::ostream& operator<<(std::ostream& os, const emitter& e) {
        const char* elem_sep = " ";
        const char* end_sep = " ";
        os << "[";
        for (const auto& elem : e.v_)
        {
            os << elem_sep << emit(elem);
            elem_sep = ", ";
        }

        return os << end_sep << "]";
    }

    const arg_type& v_;
};


int main() {
    // build test data
    std::map<std::string, std::vector<std::string>> data;

    data.emplace("a", std::vector<std::string>{ "now", "is", "the", "time" });
    data.emplace("b", std::vector<std::string>{ "for", "all", "good", "men" });
    data.emplace("c", std::vector<std::string>{ "to", "come", "to", "the" });
    data.emplace("d", std::vector<std::string>{ "aid", "of", "their", "party" });

    // request an emitter manipulator
    std::cout << emit(data) << std::endl;
}

预期输出：

{
    a: [ now, is, the, time ]
    b: [ for, all, good, men ]
    c: [ to, come, to, the ]
    d: [ aid, of, their, party ]
}

Answer 2

所以这是一个operator<<，它将从您的地图中打印出一对的内容：

std::ostream& operator<<(std::ostream& out, const std::pair<std::string, std::vector<std::string>>& p) {
    out << p.first << ": "; // prints the string from key
    for (const auto& i : p.second) // loops throught the whole vector that is asociated with that key
        out << i << ", ";
    return out;
}

所以在这个例子中使用它。如果您将其输入到地图中：

std::map<std::string, std::vector<string>> data;
std::vector<std::string> vec = {"VAL1", "VAL2", "VAL3"};
data.insert(std::make_pair("KEY", vec));
auto it = data.find("KEY");
std::cout << *it;

这将是使用上面的运算符

KEY: VAL1, VAL2, VAL3, 

您还可以稍微更改格式，以便逗号不在最后一个值之后，但这只是一个外观问题。你的问题是你想打印向量，而它没有标准运算符

Answer 3

要自定义打印矢量，您必须自己编写一些代码。为确保使用您的自定义运算符，我建议您使用 std::stringstream 将您的键值对转换为字符串，然后将其输入到 std::ostream_iterator<std::string>。

一些东西，比如this（请原谅using namespace std;）：

#include <iostream>
#include <vector>
#include <string>
#include <iterator>
#include <algorithm>
#include <sstream>
#include <map>

using namespace std;

int main() {
    map<string, vector<string>> m {
        {"a", {"1", "2"}},
        {"b", {"1", "2"}},
        {"b", {"1", "2"}},
    };

    transform(begin(m), end(m), ostream_iterator<string>(cout), [](auto& p){
        stringstream ss;
        ss << p.first << ", {";
        bool first = true;
        for (auto& s : p.second)
        {
          ss << (first ? "" : ", ") << s;
          first = false;
        }
        ss << "}\n";
        return ss.str();
    });

    return 0;
}

operator<< 实际上并不是我写的，但你可以用你的来代替你的 lambda 主体）；