Can I use template or auto keyword in C++ to create a variable that I can use with std::cin to get a value from user?

Question

This line of code is wrong and won't compile, and I am wondering how it can be fixed.

#include <iostream>

template <typename T>

auto getValue()
{
    std::cout << "Enter an integral value: ";
    T value{};
    std::cin >> value;
    // I would do input handing later
    return value;
}

int main()
{
    auto value1{getValue()};
    std::cout << value1 << '\n';
}

Please point out if there is a way to make it possible or if it is not possible then why?

I want to receive an integral value from the user and create a variable depending on the value that they entered.

For example, if they entered 56, then an integer variable will be created. And if they entered 56.78, then a double variable will be created since it is a fractional value.

Answer 1

Types are fixed at compile time so your code can't work. If you really need to do this then there are a few approaches.

1. The javascript approach of just using double everywhere, a double can hold 53 bit integers without losing precision.
2. Return a std::variant

Option 2 could look something like this:

#include <variant>
#include <iostream>
#include <string>
#include <optional>

std::optional<int> parseInt(const std::string& str)
{
    try
    {
        size_t pos;
        int value = std::stoi(str, &pos);
        if (pos == str.size())
        {
            return value;
        }
    }
    catch (std::exception&)
    {
    }
    return {};
}

std::optional<double> parseDouble(const std::string& str)
{
    try
    {
        size_t pos;
        double value = std::stod(str, &pos);
        if (pos == str.size())
        {
            return value;
        }
    }
    catch (std::exception&)
    {
    }
    return {};
}
std::variant<int, double> getValue()
{
  while (true)
  {
    std::cout << "Enter a numerical value: ";
    std::string str;
    std::getline(std::cin, str);
    auto i = parseInt(str);
    if (i)
    {
        return *i;
    }
    auto d = parseDouble(str);
    if (d)
    {
        return *d;
    }
    throw std::invalid_argument("invalid input");
  }
}