c++ - Why is a vector of pointers not castable to a const vector of const pointers?

Question

The type vector<char *> is not convertible to const vector<const char*>. For example, the following gives a compilation error:

#include <vector>

using namespace std;

void fn(const vector<const char*> cvcc)
{
}

int main()
{
    vector<char *> vc = vector<char *>(); 

    fn(vc);
}

I understand why vector<char*> is not convertable to vector<const char*> - extra members of type const char * may be added to the vector, and afterwards they would be accessible as non-const. However, if the vector itself is const, this can't happen.

My best guess is that this would be harmless, but there is no way the compiler is allowed to deduce that this would be harmless.

How can this be worked around?

This question was suggested by the C++ FQA here.

Answer 1

In general, C++ does not allow you to cast someclass<T> to someclass<U> as a template someclass might be specialized for U. It doesn't matter how T and U are related. This mean that the implementation and thus the object layout might be different.

Sadly, there is no way to tell the compiler in which cases the layout and/or behaviour didn't change and the cast should be accepted. I imagine it could be very useful for std::shared_ptr and other use-cases, but that is not going to happen anytime soon (AFAIK).