Why Does Subtracting 1 from an Empty Vector's Size in C Result in a Large Positive Number Instead of -1?

Vector Size Discrepancy with Empty Vector in C

Consider the following code:

#include <vector>
#include <iostream>
using namespace std;
int main() {
    vector<int> value;
    cout << value.size() << endl;  // output 0
    cout << value.size() - 1 << endl;  // output 18446744073709551615
}

The code prints 0 for the first cout, indicating an empty vector as expected. However, the second cout prints a large positive number (18446744073709551615). Why isn't it -1, as one might intuitively expect?

Understanding Unsigned Types

To understand this behavior, we need to consider the type of the vector size function. vector::size() returns a size_t value, which is an unsigned data type. Unsigned data types can only represent positive numbers and 0. Negative numbers cannot be represented with these types.

When we subtract 1 from the size of an empty vector, we obtain the -1 value as expected. However, because size_t is unsigned, the -1 value overflows to the largest positive value this data type can represent, which is 18446744073709551615 in this case.

Therefore, the second cout statement displays the result of this overflow, rather than the expected -1 value.

The above is the detailed content of Why Does Subtracting 1 from an Empty Vector's Size in C Result in a Large Positive Number Instead of -1?. For more information, please follow other related articles on the PHP Chinese website!