Memory allocation for objects in a vector

Question

I have a structure coord and a vector containing objects of type coord :

struct coord { int x1; int x2; }; vector<coord> v[n];

Now when I try to put something(just after vector declaration) into vector v using v[0].x1=2 then compiler gives an error saying

'class std::vector<coord, std::allocator<coord> > has no member named x1'

but when I use a temp object of coord type to store coordinates, define vector like

vector<coord> v //i.e without specifying size of vector

,push it into vector and then try to access v[0].x1, it works fine.

So why I am not able to put into vector using first way but second way?

John Kugelman · Accepted Answer · 2013-02-26 04:15:50Z

To create a vector of size n use parentheses, not square brackets.

vector<coord> v(n);

Using brackets creates an array of n vectors rather than a vector with n coordinates.

Ed Swangren · Accepted Answer · 2013-02-26 05:01:12Z

3

You declared an array of vectors, not a single vector, so v[n] returns a vector. You should have called the constructor with a size_t argument.

vector<coord> v(size);

edited Feb 26, 2013 at 5:01

answered Feb 26, 2013 at 4:16

Ed Swangren

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2 Comments

us2012 Over a year ago

capacity and size has a different meanings in the context of std::vector, and what the vector<T> (size_type n) constructor does is create a vector of size n filled with n default-constructed elements. On the other hand, a vector with capacity c may well contain less than c elements. I know you are aware of that and using it as a variable name only but I think in this context it would be good to change it to avoid confusion later.

Ed Swangren Over a year ago

@us2012: You're right, it was a sloppy use of the word capacity. I'll fix it.

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