Allocate memory for a vector

Question

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..

Do you have any code you could show? The last line of your question really confuses me — default, Dec 13 '10 at 10:15
Yes, it sounds like you have made a mistake in your code. You probably shouldn't have to reserve space manually — , Dec 13 '10 at 10:27

Frédéric Hamidi · Accepted Answer · 2013-08-28T12:44:37.033

84

An std::vector manages its own memory. You can use the reserve() and resize() methods to have it allocate enough memory to fit a given amount of items:

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.

edited Aug 28 '13 at 12:44

answered Dec 13 '10 at 10:16

Frédéric Hamidi

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to complete this post, `resize()` adds elements constructed with their default constructor (i.e. if the type is `Elem`, then `Elem()`) – swegi Dec 13 '10 at 11:08
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@swegi, by default, yes. `resize()` can also take a second argument which will be copied into all the new elements. – Frédéric Hamidi Dec 13 '10 at 11:14
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But, if you're just using `.push_back()`, do you have to reallocate/resize? – Charles Dec 10 '15 at 14:29
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@c650 If you know how many items there will be and this amount is not a small one then it's better to allocate enough space. – Zbyszek Jul 04 '16 at 06:26
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In C++11 you can just pass count in contructor: `std::vector(1000)`. – Shital Shah Aug 23 '18 at 05:36

score 9 · Answer 2 · edited May 23 '17 at 12:26

Take a look at this You use 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.

For example:

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;

Allocate memory for a vector

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