vector and const

Question

Consider this

 void f(vector& p)
 {
 }
 int main()
 { 
  vector nonConstVec;
  f(nonConstVec);
 }

The followi

Answer 1

I've added a few lines to your code. That's sufficient to make it clear why this is disallowed:

void f(vector<const T*>& p)
 {
    static const T ct;
    p.push_back(&ct); // adds a const T* to nonConstVec !
 }
 int main()
 { 
  vector<T*> nonConstVec;
  f(nonConstVec);
  nonConstVec.back()->nonConstFunction();
 }

Answer 2

Think of like this:

You have two class like this:

class V  { T*       t;};
class VC { T const* t;};

Do you expect these two classes to be convertible automatically?
This is basically what a template class is. Each variation is a completely new type.

Thus vector<T*> and vector<T const*> are completely different types.

My first question is do you really want to store pointers?

If yes, I would suggest looking at boost::ptr_container. This holds pointers and deletes them when the vector is destroyed. But more importantly it treats the contained pointers as a normal std:vector treats its contained objects. Thus by making the vector const you can only access its members as const

void function(boost::ptr_vector<T> const& x)
{
     x.push_back(new T);  // Fail x is const.
     x[4].plop();         // Will only work if plop() is a const member method.
}

If you don't need to store pointers then store the objects (not the pointers) in the container.

void function(std::vector<T> const& x)
{
     x.push_back(T());    // Fail x is const.
     x[4].plop();         // Will only work if plop() is a const member method.
}

Answer 3

vector<T> and vector<const T> are unrelated types. The fact that T can be converted to const T doesn't mean a thing here.

You have to think about it from a type system's standpoint. Instantiated vector<int> doesn't have anything in common with vector<const int>.

Answer 4

As others have said, conversions aren't applied to the template parameters. Put another way,

vector<T>

...and:

vector<const T>

... are completely different types.

If you are trying to implement const-correctness in regard to f() not modifying the contents of the vector, this might be more along the lines of what you're looking for:

void f(vector<T>::const_iterator begin, vector<T>::const_iterator end)
{
  for( ; begin != end; ++begin )
  {
    // do something with *begin
  }
}

int main()
{
  vector<T> nonConstVec;
  f(nonConstVec.begin(), nonConstVec.end());
}