为什么这么多的电话复制缺点,我只希望只有最后九个?甚至根本不会返回价值优化。为什么这么多的复制,而转换/复制矢量
struct C
{
int _i;
C(int i) : _i(i) {}
C(const C& other) { cout << "copy cons from " << other._i << " to " << _i << endl; _i = other._i; }
};
int _tmain(int argc, _TCHAR* argv[])
{
vector<int> vi{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };
vector<C> vc;
transform(vi.begin(), vi.end(), back_inserter(vc),
[](int i)
{
return C(i);
});
}
输出:
copy cons from 1 to - 842150451
copy cons from 1 to - 842150451
copy cons from 2 to - 842150451
copy cons from 1 to - 842150451
copy cons from 2 to - 842150451
copy cons from 3 to - 842150451
copy cons from 1 to - 842150451
copy cons from 2 to - 842150451
copy cons from 3 to - 842150451
copy cons from 4 to - 842150451
copy cons from 1 to - 842150451
copy cons from 2 to - 842150451
copy cons from 3 to - 842150451
copy cons from 4 to - 842150451
copy cons from 5 to - 842150451
copy cons from 6 to - 842150451
copy cons from 1 to - 842150451
copy cons from 2 to - 842150451
copy cons from 3 to - 842150451
copy cons from 4 to - 842150451
copy cons from 5 to - 842150451
copy cons from 6 to - 842150451
copy cons from 7 to - 842150451
copy cons from 8 to - 842150451
copy cons from 9 to - 842150451
谢谢,这是真的,但为什么它增长得如此缓慢,但没有例如。每次两倍?这不是很傻吗? – Yola
@Yola它靠近两个因子增长。确切的因素取决于实施。 – juanchopanza
许多实现使用接近黄金比例(〜1.62)的增长因子而不是2的主要原因是这可以防止内存碎片。请参阅[本答案](http://stackoverflow.com/a/1100426/358277)以获得更深入的解释 –