Not only is it "read only," it's not even necessarily read only:

  #include <iostream>

  void f(const int* var)
  {
    *var = 42;
  }

  int main()
  {
    int* ptr = new int(78);
    f(ptr);
    std::cout << *ptr << std::endl;
    return 0;
  }

Compiling produces:

  [scott_s@local Code] g++ const.cpp 
  const.cpp: In function ‘void f(const int*)’:
  const.cpp:5: error: assignment of read-only location

But if we make it:

  #include <iostream>

  void f(const int* var)
  {
    int* sneaky = const_cast<int*>(var);
    *sneaky = 42;
  }

  int main()
  {
    int* ptr = new int(78);
    f(ptr);
    std::cout << *ptr << std::endl;
    return 0;
  }

We get:

  [scott_s@local Code] g++ const.cpp 
  [scott_s@local Code] ./a.out 
  42

C++ gives us escape hatches all over the place. I think that the modern approach of compartmentalizing all escape hatches into explicitly regions of "unsafe" code, and providing no escape hatches outside of such regions, is much better.