C++向量在实践中何时动态减少其分配的大小。
我知道当插入一个已满的向量时,分配的空间会加倍,但是我不清楚何时会减少分配。经典的迟滞现象是在从1/4满的向量中移除元素时将分配大小减半。
C++向量在实践中何时动态减少其分配的大小。
我知道当插入一个已满的向量时,分配的空间会加倍,但是我不清楚何时会减少分配。经典的迟滞现象是在从1/4满的向量中移除元素时将分配大小减半。
如果没有明确的指示,它不会缩小已分配的内存。
C++11中有一个shrink_to_fit
方法,可以要求实现这样做,但它可能不会减少已分配的内存。在之前的版本中,您必须创建一个新副本并交换旧副本。
shrink_to_fit
方法,也不能保证一定会减小容量(这只是一个非强制性的请求)。 - vsoftcostd::vector<T>(vec).swap(vec);
。 - Drew Dormann std::vector<int> v;
for(unsigned x=0;x<20;++x)
{
v.push_back(x);
out << "elements: " << v.size() << ", capacity: " << v.capacity() << std::endl;
}
for(unsigned x=v.size();x>0;--x)
{
v.pop_back();
out << "elements: " << v.size() << ", capacity: " << v.capacity() << std::endl;
}
elements: 1, capacity: 1
elements: 2, capacity: 2
elements: 3, capacity: 4
elements: 4, capacity: 4
elements: 5, capacity: 8
elements: 6, capacity: 8
elements: 7, capacity: 8
elements: 8, capacity: 8
elements: 9, capacity: 16
elements: 10, capacity: 16
elements: 11, capacity: 16
elements: 12, capacity: 16
elements: 13, capacity: 16
elements: 14, capacity: 16
elements: 15, capacity: 16
elements: 16, capacity: 16
elements: 17, capacity: 32
elements: 18, capacity: 32
elements: 19, capacity: 32
elements: 20, capacity: 32
elements: 19, capacity: 32
elements: 18, capacity: 32
elements: 17, capacity: 32
elements: 16, capacity: 32
elements: 15, capacity: 32
elements: 14, capacity: 32
elements: 13, capacity: 32
elements: 12, capacity: 32
elements: 11, capacity: 32
elements: 10, capacity: 32
elements: 9, capacity: 32
elements: 8, capacity: 32
elements: 7, capacity: 32
elements: 6, capacity: 32
elements: 5, capacity: 32
elements: 4, capacity: 32
elements: 3, capacity: 32
elements: 2, capacity: 32
elements: 1, capacity: 32
elements: 0, capacity: 32
vector<int> x;
x.resize(500);
x = vector<int>(); // assigning a copy of new empty vector will shrink memory usage