C ++中向量的.size（）实际上做了什么？

Question

换句话说，.size（）计算向量对象的每个元素，只要它被调用并返回该值，或者向量对象是否具有（size_t？）成员，该成员保存当前在向量中的元素数量，以及值这个成员是由.size（）返回的？

Answer 1

What exactly a call to std::vector::size does depends on the particular implementation of the standard library. However, the standard places several constraints on what it can and cannot do. In particular, it requires a call to size to execute in constant time, which means it cannot count the elements (that would be linear in the container size, not constant).

A vector's implementation needs one pointer to point to the beginning of the vector, and then other two pieces of information: how large the vector's data currently is (how many elements in the vector), and how large the vector's capacity currently is (how large is the allocated data buffer). The latter two can be implemented either as pointers, or as offsets from the beginning pointer. Either representation can be used to obtain the size of data in constant time: either return the offset directly, or compute it as end - beginning .

You can verify the complexity requirement in the standard itself, or in suitable reference documentation .

Answer 2

std::vector::size的实现依赖于实现，即使它需要在O(1)具有复杂性。