vector size - 1 when size is 0 in C++

Question

The following code

#include 
#include 
using namespace std;
int main() {
    vector value;
    cout << value.         


        

Answer 1

Unsigned integer types in C++ do “wrap around arithmetic” a.k.a. clock arithmetic a.k.a. modulo arithmetic. And the result of any standard library size function is unsigned, usually the type size_t. And so, when you subtract 1 from 0 of type size_t, you get the largest size_t value.

To avoid these problems you can include and define

using Size = ptrdiff_t;

and further (the second function here requires inclusion of ),

template< class Type >
auto n_items( Type const& o )
    -> Size
{ return o.size(); }

template< Size n >
auto n_items( std::bitset const& o )
    -> Size
{ return o.count(); }       // Corresponds to std::set::size()

Then you can write

n_items( v )

and get a signed integer result, and -1 when you subtract 1 from 0.