I'm trying to access the content of a variant. I don't know what's in there, but thankfully, the variant does. So I thought I'll just ask the variant what index it is on and then use that index to std::get its content.
But this does not compile:
#include <variant>
int main()
{
std::variant<int, float, char> var { 42.0F };
const std::size_t idx = var.index();
auto res = std::get<idx>(var);
return 0;
}
The error happens in the std::get call:
error: no matching function for call to ‘get<idx>(std::variant<int, float, char>&)’
auto res = std::get<idx>(var);
^
In file included from /usr/include/c++/8/variant:37,
from main.cpp:1:
/usr/include/c++/8/utility:216:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(std::pair<_Tp1, _Tp2>&)’
get(std::pair<_Tp1, _Tp2>& __in) noexcept
^~~
/usr/include/c++/8/utility:216:5: note: template argument deduction/substitution failed:
main.cpp:9:31: error: the value of ‘idx’ is not usable in a constant expression
auto res = std::get<idx>(var);
^
main.cpp:7:15: note: ‘std::size_t idx’ is not const
std::size_t idx = var.index();
^~~
How can I fix this?
Essentially, you cannot.
You wrote:
I don't know what's in there, but thankfully, the variant does
... but only at run-time, not at compile-time.
And that means your idx value is not compile-time.
And that means you can't use get<idx>() directly.
Something you could do is have a switch statement; ugly, but it would work:
switch(idx) {
case 0: { /* code which knows at compile time that idx is 0 */ } break;
case 1: { /* code which knows at compile time that idx is 1 */ } break;
// etc. etc.
}
This is rather ugly however. As comments suggest, you might as well std::visit() (which is not very different from the code above, except using variadic template arguments instead of being this explicit) and avoid the switch altogether. For other index-based approaches (not specific to std::variant), see: