C++ stack-based object allocation

Question

In C++ there are two ways one can declare an object. For example:

// The first way
vector<int> *nums = new vector<int>;

// The second way
vector<int> nums;

People say that the first declaration allocates the object in the heap and the second on the stack. I can imagine how it works if the vector object is in the heap. The compiler would just find a free block in the heap to store the vector. But what would happen if the object is allocated on the stack as I keep pushing new elements to the vector? Will there be enough memory space? If not, how would the compiler find a sufficiently big memory block on the stack to store the vector when the size of the vector can change?

Great beginners question, and nicely stated.

Johan Lundberg
– Johan Lundberg

2015-03-18 21:59:28 +00:00
Commented Mar 18, 2015 at 21:59 — Johan Lundberg
– Johan Lundberg, Commented Mar 18, 2015 at 21:59

myaut · Accepted Answer · 2015-03-18 21:26:48Z

10

Putting vector object on stack doesn't mean it will put its elements on stack. Check documentation:

Internally, vectors use a dynamically allocated array to store their elements. This array may need to be reallocated in order to grow in size when new elements are inserted, which implies allocating a new array and moving all elements to it.

From: http://www.cplusplus.com/reference/vector/vector/

answered Mar 18, 2015 at 21:26

myaut

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1 Comment

Zig Mandel Over a year ago

Yes. Only the "wrapper" vector is in the stack. Internally it probably just contains a pointer. It would be different if it was an array, where the array size must remain fixed precisely because of the issues you ask.

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