How to define a templated variable which is inside a class

Question

I have two files quantity.h and main.cpp whose contents are below

file quantity.h

#include <iostream>
#include <utility>
template <typename T = unsigned int>
class quantity
{
    enum volume {ltr,gallon,oz};

    public:
    using ret = std::pair<volume,T>;
    ret get_volume()
    {
        return std::make_pair(ltr,5);
    }
    
}; 

Then I have main.cpp

#include<quantity.h>
int main() {
    quantity <unsigned int> q1;
    ret t = q1.get_volume();

    return 0;
}

It compiles if I only call q1.get_volume();but I want to store the result in a std::pair ret which is declared/defined in quantity.h file . I get compilation error for the code as

error: 'ret' was not declared in this scope

Answer 1

Use auto, and let the compiler do the deduction work for you :

#include <iostream>
#include <utility>

template <typename T = unsigned int>
class quantity
{
    enum volume { ltr, gallon, oz };

public:

    auto get_volume()
    {
        return std::make_pair(ltr, 5);
    }

};

int main() 
{
    quantity <unsigned int> q1;
    auto t = q1.get_volume();
    return 0;
}