std::vector versus std::array in C++

What are the difference between a std::vector and an std::array in C++? When should one be preferred over another? What are the pros and cons of each? All my textbook does is list how they are the same.

std::vector is a template class that encapsulate a dynamic array¹, stored in the heap, that grows and shrinks automatically if elements are added or removed. It provides all the hooks (begin(), end(), iterators, etc) that make it work fine with the rest of the STL. It also has several useful methods that let you perform operations that on a normal array would be cumbersome, like e.g. inserting elements in the middle of a vector (it handles all the work of moving the following elements behind the scenes).

Since it stores the elements in memory allocated on the heap, it has some overhead in respect to static arrays.

std::array is a template class that encapsulate a statically-sized array, stored inside the object itself, which means that, if you instantiate the class on the stack, the array itself will be on the stack. Its size has to be known at compile time (it's passed as a template parameter), and it cannot grow or shrink.

It's more limited than std::vector, but it's often more efficient, especially for small sizes, because in practice it's mostly a lightweight wrapper around a C-style array. However, it's more secure, since the implicit conversion to pointer is disabled, and it provides much of the STL-related functionality of std::vector and of the other containers, so you can use it easily with STL algorithms & co. Anyhow, for the very limitation of fixed size it's much less flexible than std::vector.

For an introduction to std::array, have a look at this article; for a quick introduction to std::vector and to the the operations that are possible on it, you may want to look at its documentation.

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1. Actually, I think that in the standard they are described in terms of maximum complexity of the different operations (e.g. random access in constant time, iteration over all the elements in linear time, add and removal of elements at the end in constant amortized time, etc), but AFAIK there's no other method of fulfilling such requirements other than using a dynamic array. As stated by @Lucretiel, the standard actually requires that the elements are stored contiguously, so it is a dynamic array, stored where the associated allocator puts it.

To emphasize a point made by @MatteoItalia, the efficiency difference is where the data is stored. Heap memory (required with vector) requires a call to the system to allocate memory and this can be expensive if you are counting cycles. Stack memory (possible for array) is virtually "zero-overhead" in terms of time, because the memory is allocated by just adjusting the stack pointer and it is done just once on entry to a function. The stack also avoids memory fragmentation. To be sure, std::array won't always be on the stack; it depends on where you allocate it, but it will still involve one less memory allocation from the heap compared to vector. If you have a

• small "array" (under 100 elements say) - (a typical stack is about 8MB, so don't allocate more than a few KB on the stack or less if your code is recursive)
• the size will be fixed
• the lifetime is in the function scope (or is a member value with the same lifetime as the parent class)
• you are counting cycles,

definitely use a std::array over a vector. If any of those requirements is not true, then use a std::vector.

Using the std::vector<T> class:

• ...is just as fast as using built-in arrays, assuming you are doing only the things built-in arrays allow you to do (read and write to existing elements).

• ...automatically resizes when new elements are inserted.

• ...allows you to insert new elements at the beginning or in the middle of the vector, automatically "shifting" the rest of the elements "up"( does that make sense?). It allows you to remove elements anywhere in the std::vector, too, automatically shifting the rest of the elements down.

• ...allows you to perform a range-checked read with the at() method (you can always use the indexers [] if you don't want this check to be performed).

There are two three main caveats to using std::vector<T>:

1. You don't have reliable access to the underlying pointer, which may be an issue if you are dealing with third-party functions that demand the address of an array.

2. The std::vector<bool> class is silly. It's implemented as a condensed bitfield, not as an array. Avoid it if you want an array of bools!

3. During usage, std::vector<T>s are going to be a bit larger than a C++ array with the same number of elements. This is because they need to keep track of a small amount of other information, such as their current size, and because whenever std::vector<T>s resize, they reserve more space then they need. This is to prevent them from having to resize every time a new element is inserted. This behavior can be changed by providing a custom allocator, but I never felt the need to do that!

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Edit: After reading Zud's reply to the question, I felt I should add this:

The std::array<T> class is not the same as a C++ array. std::array<T> is a very thin wrapper around C++ arrays, with the primary purpose of hiding the pointer from the user of the class (in C++, arrays are implicitly cast as pointers, often to dismaying effect). The std::array<T> class also stores its size (length), which can be very useful.

If you are considering using multidimensional arrays, then there is one additional difference between std::array and std::vector. A multidimensional std::array will have the elements packed in memory in all dimensions, just as a c style array is. A multidimensional std::vector will not be packed in all dimensions.

Given the following declarations:

int cConc[3][5];
std::array<std::array<int, 5>, 3> aConc;
int **ptrConc;      // initialized to [3][5] via new and destructed via delete
std::vector<std::vector<int>> vConc;    // initialized to [3][5]

A pointer to the first element in the c-style array (cConc) or the std::array (aConc) can be iterated through the entire array by adding 1 to each preceding element. They are tightly packed.

A pointer to the first element in the vector array (vConc) or the pointer array (ptrConc) can only be iterated through the first 5 (in this case) elements, and then there are 12 bytes (on my system) of overhead for the next vector.

This means that a std::vector> array initialized as a [3][1000] array will be much smaller in memory than one initialized as a [1000][3] array, and both will be larger in memory than a std:array allocated either way.

This also means that you can't simply pass a multidimensional vector (or pointer) array to, say, openGL without accounting for the memory overhead, but you can naively pass a multidimensional std::array to openGL and have it work out.

Summarizing the above discussion in a table for quick reference:

	C-Style Array	std::array	std::vector
Size	Fixed/Static	Fixed/Static	Dynamic
Memory efficiency	More efficient	More Efficient	Less efficient (May double its size on new allocation.)
Copying	Iterate over elements or use std::copy()	Direct copy: a2 = a1;	Direct copy: v2 = v1;
Passing to function	Passed by pointer. (Size not available in function)	Passed by value	Passed by value (Size available in that function)
Size	sizeof(a1) / sizeof(a1[0])	a1.size()	v1.size()
Use case	For quick access and when insertions/deletions not frequently needed.	Same as classic array but safer and easier to pass and copy.	When frequent additions or deletions might be needed

A vector is a container class while an array is an allocated memory.