C++ how to cast std::vector to const std::vector?

Question

I want to cast a std::vector<int> to const std::vector<const int>, it seems that it can not be automatically casted. So I have some questions:

1. I can easily cast std::vector<int> to const std::vector<int>, why is that?

2. If I want to cast to const std::vecor<cosnt int>, how should I do that? I've tried const_cast but doesn't work

Answer 1

If I want to cast to const std::vecor<cosnt int>, how should I do that? I've tried const_cast but doesn't work

Short answer: Don't.

When you make a std::vector<int> const, as-in const std::vector<int>, the contents themselves become implicitly const as well. In other words, if you write something like this, you won't be able to modify the elements:

const std::vector<int> values{1,2,3,4,5};

//Nope
//values.emplace_back(6);

//Also Nope
//values[3] = 5;

//Still nope
//values.erase(values.begin() + 1, values.begin() + 3);

//Nuh-uh
//std::vector<int> & mutable_values = values;

//This, however, is okay.
std::vector<int> const& immutable_values = values;

//Same restrictions though
//immutable_values[2] = 6;
//immutable_values.emplace_back(7);

//This is fine
std::vector<int> copy_of_values = values;

//But that's because you made a copy
copy_of_values.emplace_back(6);
assert(copy_of_values.size() != values.size());
assert(copy_of_values != values);

This is why STL containers like std::vector, std::list, std::map, and so on, prohibit the use of const members within their template parameter lists: because making the container itself const also makes its contents const, which is a stated contract of the design of these containers. Some "containers" don't have this property, like Smart Pointers, which is why you'll sometimes see stuff like this:

std::shared_ptr<int> ptr = std::make_shared<int>(42);
std::shared_ptr<const int> non_modifying_ptr = ptr;

Which is part of the core functionality of reference-counted pointers.

This, incidentally, is all part of making sure your code is "const-correct", and I strongly advise you do a google search on that very subject and learn about what it is, and how to properly apply it in your code to make your code safer and more performant.