Template and deque [duplicate] - c++

Question

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Why can templates only be implemented in the header file?
(17 answers)
Closed 2 years ago.
I'm trying to work with deque and template and I wanted to add others files (func.cpp and func.h) so I can declare all my function in these files and then include func.h in main.cpp but I got an some errors:
1. argument list for variable template "display" is missing in main.cpp
2.'deque': undeclared identifier in func.cpp
3.'d': undeclared identifier in func.cpp
here's the main.cpp:
#include <iostream>
#include <algorithm>
#include <deque>
#include "funcr.h"
using namespace std;
int main()
{
deque <int> d1{ 1,5,3,9 };
d1.push_front(2);
display(d1);
return 0;
}
and here's the func.cpp:
#include <iostream>
#include <deque>
template<typename T>
void display(deque<T> d)
{
for (auto e : d)
std::cout << e << std::endl;
}
and there is the func.h:
#pragma once
template<typename T>
void display(deque<T>d);
can someone help me please?

Answer 1

You must define display in func.h, because templated functions must be implemented in header files unless they are explicitly instantiated.
Also, you did not write using namespace std; in func.h (not that you should write it anywhere, especially headers). As a result, you need to write std::deque instead of merely deque.
So func.h should look like the following:
#include <iostream>
#include <deque>
template<typename T>
void display(const std::deque<T>& d)
{
for (auto e : d)
std::cout << e << std::endl;
}