How to overload the friend extraction operator (>>) for templates in C++?
I am trying to overload the friend >> operator with templates. I don't want to define it inline.
I had tried to do the same with the help of the method add() defined in the code below. It works fine. I would like my >> operator to do the same.
The following is my code:
#include<iostream>
template<class T>class Demo;
template<class T>
std::ostream& operator<<(std::ostream&, const Demo<T> &);
template<class T>
std::istream& operator>>(std::istream&, const Demo<T> &);
template<class T>
class Demo {
private:
T data; // To store the value.
public:
Demo(); // Default Constructor.
void add(T element); // To add a new element to the object.
Demo<T> operator+(const Demo<T> foo);
friend std::ostream& operator<< <T>(std::ostream &out, const Demo<T> &d);
friend std::istream& operator>> <T>(std::istream &in, const Demo<T> &d);
};
template<class T>
Demo<T>::Demo() {
data = 0;
}
template<class T>
void Demo<T>::add(T element) {
data = element;
}
template<class T>
Demo<T> Demo<T>::operator+(const Demo<T> foo) {
Demo<T> returnObject;
returnObject.data = this->data + foo.data;
return returnObject;
}
template<class T>
std::ostream& operator<<(std::ostream &out, const Demo<T> &d) {
out << d.data << std::endl;
return out;
}
template<class T>
std::istream& operator>>(std::istream &in, const Demo<T> &d) {
in >> d.data;
return in;
}
int main() {
Demo<int> objOne;
std::cin>>objOne;
Demo<int>objTwo;
objTwo.add(3);
Demo<int>objThree = objOne + objTwo;
std::cout << "Result = " << objThree;
return 0;
}
Actual Issue
While trying to overload the friend extraction operator ( >> ), the compiler shows the error as follows:
testMain.cpp:52:15: required from here
testMain.cpp:46:8: error: no match for 'operator>>' (operand types are 'std::istream {aka std::basic_istream}' and 'const int')
in >> d.data;
^
Expected Output
Result = 59 RUN SUCCESSFUL (total time: 49ms)
References
"overloading the extraction operator >> in C++ [duplicate]" defines how to overload the >> operator. But it doesn't mention it's usage with templates.
"Overloading friend operator << for template class" defines how to overload the << operator, but not the >> .
"Overloading Extraction operator" also doesn't cover the template concept.
The problem has nothing to do with templates.
operator>> modifies the data on the right-hand side, but you declared that parameter as const , so the operator cannot modify it. The compiler error even states that the value to be modified ( d.data ) is const:
testMain.cpp:46:8: error: no match for 'operator>>' (operand types are 'std::istream {aka std::basic_istream}' and 'const int' )
You need to remove the const from the second parameter:
template<class T>
std::istream& operator>>(std::istream&, Demo<T> &);
...
template<class T>
class Demo {
...
public:
...
friend std::istream& operator>> <T>(std::istream &in, Demo<T> &d);
};
...
template<class T>
std::istream& operator>>(std::istream &in, Demo<T> &d) {
in >> d.data;
return in;
}
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