vector.size（）在比较中意外工作

Question

When I use vector.size() in comparisions it gives unexpected results

vector<int> v;
for(int i = 0; i < v.size() -1;++i){
    printf("i = %d\n", i);
    printf("v[i] = %d\n", v[i]);
}

since the size of vector is 0, it shoudn't print anything but, it enters for loop and prints i = 0 and give segmentation fault. But it shouldn't even enter the for loop as v.size() - 1 is -1 .

Why is it happening?

Answer 1

The problem is your loop:

 for(int i = 0; i < v.size() -1;++i)

More specifically, this part of the condition: v.size() - 1 .

The size function returns a value of type size_type , which if you read eg this vector reference will see is an unsigned type.

That means when you subtract 1 from the value 0 , you don't get -1 but instead get a very large value since unsigned underflow wraps around to its highest value.

That means your loop will indeed iterate, at least once, and lead to UB (Undefined Behavior) when you index out of bounds.

Answer 2

向量大小是一个无符号的整数，因此v.size（）-1永远不会为-1，而是一些非常大的整数。