Arrays vs Vectors Introductory Similarities and Differences

Question

What are the differences between an array and a vector in C++? An example of the differences might be included libraries, symbolism, abilities, etc.

Array

Arrays contain a specific number of elements of a particular type. So that the compiler can reserve the required amount of space when the program is compiled, you must specify the type and number of elements that the array will contain when it is defined. The compiler must be able to determine this value when the program is compiled. Once an array has been defined, you use the identifier for the array along with an index to access specific elements of the array. [...] arrays are zero-indexed; that is, the first element is at index 0. This indexing scheme is indicative of the close relationship in C++ between pointers and arrays and the rules that the language defines for pointer arithmetic.

— C++ Pocket Reference

Vector

A vector is a dynamically-sized sequence of objects that provides array-style operator[] random access. The member function push_back copies its arguments via copy constructor, adds that copy as the last item in the vector and increments its size by one. pop_back does the exact opposite, by removing the last element. Inserting or deleting items from the end of a vector takes amortized constant time, and inserting or deleting from any other location takes linear time. These are the basics of vectors. There is a lot more to them. In most cases, a vector should be your first choice over a C-style array. First of all, they are dynamically sized, which means they can grow as needed. You don't have to do all sorts of research to figure out an optimal static size, as in the case of C arrays; a vector grows as needed, and it can be resized larger or smaller manually if you need to. Second, vectors offer bounds checking with the at member function (but not with operator[]), so that you can do something if you reference a nonexistent index instead of simply watching your program crash or worse, continuing execution with corrupt data.

— C++ Cookbook

User · Answer

I ll add that arrays are very low-level constructs in C   and you should try to stay away from them as much as possible when  learning the ropes  -- even Bjarne Stroustrup recommends this  he s the designer of C        Vectors come very close to the same performance as arrays  but with a great many conveniences and safety features  You ll probably start using arrays when interfacing with API s that deal with raw arrays  or when building your own collections

User · Answer

arrays    are a builtin language construct  come almost unmodified from C89  provide just a contiguous  indexable sequence of elements  no bells and whistles  are of fixed size  you can t resize an array in C    unless it s an array of POD and it s allocated with malloc   their size must be a compile-time constant unless they are allocated dynamically  they take their storage space depending from the scope where you declare them  if dynamically allocated  you must explicitly deallocate them  if they are dynamically allocated  you just get a pointer  and you can t determine their size  otherwise  you can use sizeof  hence the common idiom sizeof arr  sizeof  arr   that however fails silently when used inadvertently on a pointer   automatically decay to a pointers in most situations  in particular  this happens when passing them to a function  which usually requires passing a separate parameter for their size  can t be returned from a function  can t be copied assigned directly  dynamical arrays of objects require a default constructor  since all their elements must be constructed first    std  vector    is a template class  is a C   only construct  is implemented as a dynamic array  grows and shrinks dynamically  automatically manage their memory  which is freed on destruction  can be passed to returned from functions  by value   can be copied assigned  this performs a deep copy of all the stored elements   doesn t decay to pointers  but you can explicitly get a pointer to their data   amp vec 0  is guaranteed to work as expected   always brings along with the internal dynamic array its size  how many elements are currently stored  and capacity  how many elements can be stored in the currently allocated block   the internal dynamic array is not allocated inside the object itself  which just contains a few  bookkeeping  fields   but is allocated dynamically by the allocator specified in the relevant template parameter  the default one gets the memory from the freestore  the so-called heap   independently from how where the actual object is allocated  for this reason  they may be less efficient than  regular  arrays for small  short-lived  local arrays  when reallocating  the objects are copied  moved  in C  11   does not require a default constructor for the objects being stored  is better integrated with the rest of the so-called STL  it provides the begin   end   methods  the usual STL typedefs         Also consider the  modern alternative  to arrays - std  array  I already described in another answer the difference between std  vector and std  array  you may want to have a look at it

User · Answer

Those reference pretty much answered your question. Simply put, vectors' lengths are dynamic while arrays have a fixed size. when using an array, you specify its size upon declaration:

int myArray[100];
myArray[0]=1;
myArray[1]=2;
myArray[2]=3;

for vectors, you just declare it and add elements

vector<int> myVector;
myVector.push_back(1);
myVector.push_back(2);
myVector.push_back(3);
...

at times you wont know the number of elements needed so a vector would be ideal for such a situation.

[c++] Arrays vs Vectors: Introductory Similarities and Differences

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