I have a class field which is a std::vector. I know how many elements I want this vector to contain: N. How do I initialize the vector with N elements?
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Are you referring to the initial size or the initial capacity?Emile Cormier– Emile Cormier2011-03-14 04:15:20 +00:00Commented Mar 14, 2011 at 4:15
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@Emile. Initial size I guess. I don't quite follow. Are you referring to using reserve() to reserve capacity?Himadri Choudhury– Himadri Choudhury2011-03-14 04:18:41 +00:00Commented Mar 14, 2011 at 4:18
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Yes, I am. Let me put it in another way: Do you want your vector to initially have N elements, or do you want your vector to be able to grow to N elements without reallocation?Emile Cormier– Emile Cormier2011-03-14 04:21:07 +00:00Commented Mar 14, 2011 at 4:21
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@Emile. Initially have N elements, answers below by James and Jerry were exactly what I was looking for.Himadri Choudhury– Himadri Choudhury2011-03-14 04:26:54 +00:00Commented Mar 14, 2011 at 4:26
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3 Answers
std::vector has a constructor declared as:
vector(size_type N, const T& x = T());
You can use it to construct the std::vector containing N copies of x. The default value for x is a value initialized T (if T is a class type with a default constructor then value initialization is default construction).
It's straightforward to initialize a std::vector data member using this constructor:
struct S {
std::vector<int> x;
S() : x(15) { }
}
3 Comments
Ben Voigt
You've already alluded to the fact that the N elements are copy-constructed. I don't think there's a way to create a vector in such a way that the elements are default constructed (which is how I interpret the question title), although in C++0x it can be done post-construction using
emplace_back.
James McNellis
@Ben: Well, one element is default constructed here :-). In C++0x, the constructor I mention is actually two constructors:
vector(size_type N) and vector(size_type N, const T& x). The first one value initializes the N elements and does not call any copy constructors. The resize function has similarly been split into two overloads.Ben Voigt
The parameter is default constructed, but it isn't a vector element at all. Good to know that in C++0x there's now two implementations instead of a default argument.
class myclass {
std::vector<whatever> elements;
public:
myclass() : elements(N) {}
};
Comments
All the constructors that allow you to specify a size also invoke the element's constructor. If efficiency is paramount, you can use the reserve() member function to reserve the size. This does not actually create any elements, so it is more efficient. In most cases, though, supplying the size through the vector constructor is just fine.
