Why vector.size()-1 gives garbage value? [duplicate]

问题

This question already has answers here:

How do unsigned integers work (3 answers)

Closed 2 years ago.

I had tried running this code

// vector::size
#include <iostream>
#include <vector>

int main ()
{
   std::vector<int> myints;
   std::cout << "size: " << myints.size() << '\n';
   std::cout << "size: " << myints.size()-1 << '\n';

 return 0; 
}

And Surprisingly the output came

0

garbage Value

It should be

0

-1

Here's the :code

回答1:

myints.size() is an unsigned type: formally a std::vector<int>::size_type. Subtracting 1 from an unsigned type with a value of 0 will cause wrap-around effects, in your case, to

std::numeric_limits<std::vector<int>::size_type>::max()

It would not have printed "garbage value": but the number above, which will be one less than a large power of 2.

来源：https://stackoverflow.com/questions/48646518/why-vector-size-1-gives-garbage-value