C++ display map that has vector - c++

Question

std::map<std::string, std::vector<string>> data;
In order to print out this by using copy, how should my std::ostream_iterator be?
Apparently std::ostream_iterator<std::pair<std::string, std::vector<std::string>>> out_it(std::cout, "\n"); did not make it.
My operator<< overload is the following std::ostream& operator<<(std::ostream& out, const std::pair<std::string, std::vector<std::string>>& p) and it writes out the p.first and p.second and returns it.

Answer 1

If you do any serious programming in C++, you will eventually need a generic way to print out collections.
Here is the basis of one:
#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;
}
Expected output:
{
a: [ now, is, the, time ]
b: [ for, all, good, men ]
c: [ to, come, to, the ]
d: [ aid, of, their, party ]
}

Answer 2

So here is a operator<< that will print out the contents of one pair from your map:
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;
}
So to use it in this example. If you ennter this into your map:
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;
This would be what wil get printed out using the operator<< above:
KEY: VAL1, VAL2, VAL3,
You can also change the formatting a bit so the comma isn't after the last value as well but that's only a cosmetic problem. Your problem was in that you wanted to print vector while it doesn't have std operator<<. So in order to print vector you must manually loop through it's content like in my example with the ranged for.

Answer 3

To custom print your vector, you'd have to write some code yourself. To make sure your custom operator is used, I suggest you use std::stringstream to turn your key-value pair into a string, which you'd then feed into an std::ostream_iterator<std::string>.
Something, like this (pardon the 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;
}
I didn't actually write the operator<<, but you can substitue yours to make the lambda body short);