Question

In the C++ Primer book, Chapter (3), there is the following for-loop that resets the elements in the vector to zero.

for (vector<int>::size_type ix = 0; ix ! = ivec.size(); ++ix)
ivec[ix] = 0;

Why is it using vector<int>::size_type ix = 0? Cannot we say int ix = 0? What is the benefit of using the first form on the the second?

Thanks.

Answer 1

The C++ Standard says,

 size_type  |  unsigned integral type  |  a type that can represent the size of the largest object in the
allocation model

Then it adds,

Implementations of containers
described in this International
Standard are permitted to assume that
their Allocator template parameter
meets the following two additional
requirements beyond those in Table 32.

• The typedef members pointer, const_pointer, size_type, and
  difference_type are
  required to be T*,T const*, size_t, and ptrdiff_t, respectively

So most likely, size_type is a typedef of size_t.

And the Standard really defines it as,

template <class T> 
class allocator 
{
   public:
       typedef size_t size_type;
       //.......
};

So the most important points to be noted are :

• size_type is unsigned integral, while int is not necessarily unsigned. 🙂
• it can represent the largest index, because it’s unsigned.