template
std::optional stofp(const std::string& st)
{
…
auto v = T {};
…
return ok ? v : std::optional {};
}

then you can call it either like

auto r = stofp(“12.56”);

which defaults T to double without any being used

or like

auto i = stofp(“123445”);

where the type of T is specified (here as an int)

so only when the non-defaulted type of T is needed are any required.

You can specify the default value for T by

auto rd = stofp(“12.56”);

but the empty are not needed in this case.