C++ 为向量分配内存
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Allocate memory for a vector
提问by Prasanth Madhavan
Can someone give me an example of how to allocate memory for a vector? A couple of lines is all I need. I have a vector that takes in 20-30 elements.. but when i try to cout it and compile it i only get the first couple of entries..
有人能给我一个如何为向量分配内存的例子吗？我只需要几行。我有一个包含 20-30 个元素的向量。
回答by Frédéric Hamidi
An std::vectormanages its own memory. You can use the reserve()and resize()methods to have it allocate enough memory to fit a given amount of items:
一个标准::矢量管理它自己的内存。您可以使用Reserve()和resize()方法让它分配足够的内存来容纳给定数量的项目：
std::vector<int> vec1;
vec1.reserve(30); // Allocate space for 30 items, but vec1 is still empty.
std::vector<int> vec2;
vec2.resize(30); // Allocate space for 30 items, and vec2 now contains 30 items.
回答by Simon Hughes
Take a look at thisYou use list.reserve(n);
看看这个你用list.reserve(n);
Vector takes care of its memory, and you shouldn't really need to use reserve() at all. Its only really a performance improvement if you already know how large the vector list needs to be.
Vector 负责处理它的内存，您根本不需要使用 Reserve()。如果您已经知道向量列表需要多大，它只是真正的性能改进。
For example:
例如：
std::vector<int> v;
v.reserve(110); // Not required, but improves initial loading performance
// Fill it with data
for(int n=0;n < 100; n++)
v.push_back(n);
// Display the data
std::vector<int>::iterator it;
for(it = v.begin(); it != v.end(); ++it)
cout << *it;
