vector.size()函数计算重吗？

Question

只是想知道大小函数计算代价是否昂贵？

vector<someBigType> vec;
vec.push_back(something0);
for(unsigned i = 0; i < a bigNumber; ++i)
{

    // do something ...

    // measure the size
    int size1 = vec.size();

    // A lot of push_backs (vec may grow very large)
    vec.push_back(something);

    // Or shall I just use counter++, whenever a push_back is called?

    // measure the size again
    int size2 = vec.size();

    int delta = size2-size1;

    // Use delta to do something
}

Answer 1

如果我们查看std :: vector :: size的 cppreference条目,它会说:

复杂

不变.

所以它在恒定的时间运行.这与C++标准草案一致,Table 96 — Container requirements后者列出了size()常量的复杂性.