Overload operator<< for a vector struct of pointer? - c++

Question

I am currently trying to make a overload operator<< to my vector of structs, such that I will print the content of the vector in a matrix format..
this is what I've tried so far
// This file is a "Hello, world!" in C++ language by GCC for wandbox.
#include <iostream>
#include <cstdlib>
#include <vector>
#include <algorithm>
struct element
{
std::vector<int> keys;
std::string item;
element(std::vector <int> key, std::string item)
{
keys = key;
this->item = item;
}
};
inline std::stream operator << (std::ostream &stream, const std::vector<element*> _element)
{
for (auto elements: _element)
{
for(auto item : elements)
{
std::stream << item.key << " "
}
}
}
int main()
{
std::cout << "Hello, Wandbox!" << std::endl;
std::vector<element> storage;
std::vector<int> test(10);
std::generate(test.begin(), test.end(), []() {
return rand() % 100;
});
std::string name = "test";
storage.push_back(element(test,name));
std::generate(test.begin(), test.end(), []() {
return rand() % 100;
});
storage.push_back(element(test,name));
std::generate(test.begin(), test.end(), []() {
return rand() % 100;
});
storage.push_back(element(test,name));
std::vector<element*> copy_of_storage;
for(auto elements: storage)
{
copy_of_storage.push_back(&(elements));
}
std::cout << copy_of_storage << std::endl;
}
but for some reason it is not working, and can't seem to understand what is going wrong?
https://wandbox.org/permlink/BQpqmz0HwoXgyS7t

Answer 1

There are a lot of things that are wrong in your operator<<.
First, your return type should be std::ostream& thus you must return stream. In your second loop :
for(auto item : elements)
elements is a pointer of element (the value type of your vector) so you can't iterate other it, you might want to iterate other elements->keys
Here is your fixed function :
inline std::ostream& operator<< (std::ostream &stream, const std::vector<element*> _element)
{
for (auto elements: _element)
for(auto item : elements->keys)
stream << item << " ";
return stream;
}
https://wandbox.org/permlink/qNYVx2yMg7L76NWe

Answer 2

There are quite a few things you can do to fix your code example - better naming conventions would be an important start.
The reason your code doesn't work is because of your operator << function. The following will work:
std::ostream& operator << (std::ostream &stream, std::vector<element*> _elements)
{
for (auto element : _elements)
{
for(auto item : element->keys)
{
stream << item << " ";
}
}
return stream;
}
Your working example: https://wandbox.org/permlink/FVXbLlXEAJpVbfAx
Furthermore:
Refactor your code - better variable names.
Try to use the member initialisation list in the element constructor.
And might be worth changing the ctor params to const &.
operator << should return a std::ostream&.
remove the inline keyword
in the operators << param list, you can remove the const from the std::vector<element*> since it is a copy of a vector that contains non-const pointers.