c++ - 使用 size_t 移动 vector 索引是否安全？

Question

我更喜欢使用 size_t 来处理 vector 索引。但是当移动索引时，它安全吗？例如，

size_t n = 10000000;
vector<int> v(n);

size_t i = 500000;
size_t shift = 20000;
int a = v(i - (-1) * shift); // Is this ok? what is `(-1) * shift` type, size_t?
int b = v(-shift + i); // Is this ok? what is `-shift` type, size_t?

Answer 1

取反一个unsigned 数量是一个有效的操作。 C++11 的第 5.3.2 节:

The negative of an unsigned quantity is computed by subtracting its value from 2^n , where n is the number of bits in the promoted operand. The type of the result is the type of the promoted operand.

因此，就定义的行为而言，这是“安全的”。