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

vector::size() is of type size_t which is an unsigned type, and unsigned integers can't represent negative numbers.

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 <stddef.h> and define

using Size = ptrdiff_t;

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

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

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

Then you can write

n_items( v )

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

The output is automatically casted to size_t because that's the return type of value.size(), which is an unsigned type. Hence you see an unsigned value printed.

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

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