[Solved] How to Add Element into a std::vector using std::any

Question

we are using push_back on an rvalue

No, that’s not the problem. any stores a copy of what it is given. It doesn’t reference some other object. It creates an object from what it is given. So even if you get a reference to the object in the any, it wouldn’t be a reference to vec itself.

If you want to store a reference to an object in an any, you need to do so explicitly by storing either a pointer or a reference_wrapper to that object:

using ref_type = decltype(std::ref(vec));

std::any anything = std::ref(vec);

std::any_cast<ref_type>(anything).get().push_back(4);

Note that having an any which references an object is dangerous. If the any outlives the lifetime of the referenced object, you’ll get bad things when you try to access the referenced object. This is part of why any stores copies by default.

Accepted Answer

we are using push_back on an rvalue

No, that’s not the problem. any stores a copy of what it is given. It doesn’t reference some other object. It creates an object from what it is given. So even if you get a reference to the object in the any, it wouldn’t be a reference to vec itself.

If you want to store a reference to an object in an any, you need to do so explicitly by storing either a pointer or a reference_wrapper to that object:

using ref_type = decltype(std::ref(vec));

std::any anything = std::ref(vec);

std::any_cast<ref_type>(anything).get().push_back(4);

Note that having an any which references an object is dangerous. If the any outlives the lifetime of the referenced object, you’ll get bad things when you try to access the referenced object. This is part of why any stores copies by default.