Why Does `-1 < a.size()` Evaluate to False for a `std::vector`?

std::vector Size Negation Paradox: Unveiled

In a puzzling program, we observe that the comparison -1 < a.size() evaluates to false, despite std::vector's size being inherently positive. This puzzling behavior stems from the underlying data types involved.

Understanding Unsigned Integral Types

The size member of std::vector returns an unsigned integral type. Unsigned integral types represent positive integers exclusively, lacking the ability to store negative values. When a negative value is encountered with an unsigned type, it is automatically converted to a large positive value through a process known as two's complement conversion.

Type Promotion in Comparison

In the comparison -1 < a.size(), the signed integer -1 is implicitly promoted to an unsigned integer before being compared to a.size(). This effectively transforms -1 into a large positive value, rendering the comparison false.

Illustrative Example

The following code snippet demonstrates the same behavior on fundamental types:

#include 

int main()
{
  std::cout << std::boolalpha;
  unsigned int a = 0;
  int b = -1;
  std::cout << (b < a) << "\n";  // prints false
}

In this example, the signed integer -1 is promoted to an unsigned integer before being compared to a. Therefore, the result is false.
Conclusion
In summary, the puzzling behavior observed in the original std::vector comparison arises from the type promotion of negative integers when compared to unsigned types. The negative integer is effectively converted to a large positive value, resulting in an unexpected outcome. As a best practice, avoid comparing signed and unsigned integers without explicitly casting the signed value to the unsigned type.
The above is the detailed content of Why Does `-1 < a.size()` Evaluate to False for a `std::vector`?. For more information, please follow other related articles on the PHP Chinese website!