# Modern C++ idoms and recipes

meshell
14.1K views

## std::variant

In the previous exercise we used std::optional to indicate failure. But how about a more detailed error message?

Enter template class std::variant. We could use std::variant to either hold a valid value or a detailed error.

### DIY

Refactor function differentiate to have return type std::variant<double, std::string> and once again make the tests pass. The variant should either be a valid value or an error string.

Refactor the code to use std::variant
#include <functional>
#include <sstream>
#include <string>
// symmetric difference quotient
double differentiate(std::function<double(double)> f, double h, double x)
{
return (f(x + h) - f(x - h)) / (2 * h);
}
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