The Standard Library or std in C++  is a collection of classes, functions, and definitions as per the C++ standard. The Standard Library or std is sometimes confused with the Standard Template Library or STL. STL contains many other data structure containers like queue, list, deque, and stack. However, array is a part of std and our topic of interest for this article.

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In this article, we’ll cover:

• Introduction to std::array in C++ Standard Library
• C++ std Array vs. Array
• Methods of std::array in C++
• How to Use the C++ std array
• FAQs on C++ std array

Introduction to std::array in C++ Standard Library

In C++, std::array represents a sequential container that stores arrays with fixed or static sizes. Array is a collection of homogeneous objects, and we can find its container within the Standard Library or std. 

Usage of std::array in C++

The std::array is not incredibly popular in usage, but its member functions can sometimes provide an edge over our normal array usage methods. We’ll show you how to use these important array member functions in this article.

C++ std Array vs. Array

• std::Array has friendly value semantics that helps us easily pass it to or return it from a function.
• std::Array has a convenient interface to find the size.
• std::Array is good to use with STL-style iterator-based algorithms
• Defining multi-dimensional array with std:: is more verbose than the conventional method.

Methods of std::array in C++

Here are the several methods associated with array in STD:

• [ ] Operator 
• front( )
• back( ) 
• swap( ) 
• empty( )
• at( )
• fill( )
• size( )
• max_size( )
• sizeof( )
• data( )
• cbegin( )
• cend( )

In the next section, we’ll look at the most commonly used array methods and how to use them via an example.

How to Use the C++ std array

Here, we take a look at how you can use array as a C++ STL container for a smoother coding experience:

Code

#include 
#include 
#include 
#include 

using namespace std;

int main() {

    // Some different ways to declare an array using std::array in C++

    // Basic integer array of size 5, indices from 0 to 4.
    array arrayExample1 = {1, 3, 5, 7, 9};
    // We need double brace brackets below as the construction uses aggregate initialization
    // Aggregate initialization refers to the use of brace-enclosed initializer lists for initializing all members of an aggregate
    array arrayExample2 {{2, 4, 6, 8, 10}};
    // String array of size 2, indices from 0 to 1.
    array  arrayExample3 = {"Interview", "Kickstart"};

    cout << "arrayExample1: ";
    for (auto number: arrayExample1) {
        cout << number << " ";
    }
    cout << endl;

    // Array size
    cout << "Array size using array.size() gives size in terms of the number of elements: " << arrayExample1.size() << endl;
    cout << "Array size using sizeof(array) gives size in terms of the bytes of memory: " << sizeof(arrayExample1) << endl;
    cout << "Maximum array size: " << arrayExample1.max_size() << endl;

    // Front and back most elements. If the array is empty, both 
    // front and back functions would cause undefined behavior
    cout << "Array front-most element: " << arrayExample1.front() << endl;
    cout << "Array back-most element: " << arrayExample1.back() << endl;

    cout << "Array's front-most element is located at the address: " << arrayExample1.cbegin() << endl;
    cout << "Array's front-most element has the value " << *arrayExample1.cbegin() << endl;

    cout << "Array's back-most element is located at the address: " << arrayExample1.cend() - 1 << endl;
    cout << "Array's back-most element has the value " << *(arrayExample1.cend() - 1) << endl;

    cout << "Is the array empty? (boolean, 0 = No, 1 = Yes): " << arrayExample1.empty() << endl;

    // Value at specific index 
    cout << "Array element at index 3 using *at()* is: " << arrayExample1.at(3) << endl;
    cout << "Array element at index 3 using [] is: " << arrayExample1[3] << endl;

    // The beginning of an array (address and value)
    cout << "The array starts at the memory address/address of the first element of the array: " << arrayExample1.data() << endl;
    cout << "The array starts with the value: " << *arrayExample1.data() << endl;

    // Fill method example
    cout << "Two arrays filled using fill method:\n";
    array arrayExample4;
    arrayExample4.fill("aa");
    for (int index = 0; index < arrayExample4.size(); index++) {
        cout << arrayExample4[index] << " ";
    }
    cout << endl;

    array arrayExample5;
    arrayExample5.fill(3);
    for (int index = 0; index < arrayExample5.size(); index++) {
        cout << arrayExample5[index] << " ";
    }
    cout << endl;

    // Swap method example
    array arrayExample6 = {1, 2, 3};
    array arrayExample7 = {4, 5, 6};

    cout << "arrayExample6: ";
    for (int index = 0; index < arrayExample6.size(); index++) {
        cout << arrayExample6[index] << " ";
    }
    cout << endl;
    cout << "arrayExample7: ";
    for (int index = 0; index < arrayExample7.size(); index++) {
        cout << arrayExample7[index] << " ";
    }
    cout << endl;

    arrayExample6.swap(arrayExample7);
    cout << "arrayExample6 after swapping it with arrayExample7: ";
    for (int index = 0; index < arrayExample6.size(); index++) {
        cout << arrayExample6[index] << " ";
    }
    cout << endl;

    return 0;
}

Output

arrayExample1: 1 3 5 7 9 
Array size using array.size() gives size in terms of the number of elements: 5
Array size using sizeof(array) gives size in terms of the bytes of memory: 20
Maximum array size: 5
Array front-most element: 1
Array back-most element: 9
Array's front-most element is located at the address: 0x7fff787a4590
Array's front-most element has the value 1
Array's back-most element is located at the address: 0x7fff787a45a0
Array's back-most element has the value 9
Is the array empty? (boolean, 0 = No, 1 = Yes): 0
Array element at index 3 using *at()* is: 7
Array element at index 3 using [] is: 7
The array starts at the memory address/address of the first element of the array: 0x7fff787a4590
The array starts with the value: 1
Two arrays filled using fill method:
aa aa aa aa 
3 3 3 3 3 
arrayExample6: 1 2 3 
arrayExample7: 4 5 6 
arrayExample6 after swapping it with arrayExample7: 4 5 6 

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FAQs on C++ std array

Q1. How do you initialize a single std::array in C++?

We can initialize std::array in two common ways. One method is to simply assign values to the array during declaration. For example, std::array<int, 3> intArray = {1, 3, 5}; initializes an integer type std::array with the name “intArray” of length three. Another method involves declaring the contents of the array during the declaration like this: array<int, 6> arrayExample{{2, 4, 6, 8, 10}};

Q2. Is an array an STL container?

No, array is present in the Standard Library or std, not within the Standard Template Library or STL. Arrays in std are fixed-size sequence containers that hold a fixed number of elements in a strictly linear sequence.

Q3. What is the difference between std array and C array?

std::array acts as a zero-overhead wrapper for C arrays. It allows arrays the standard value like semantics we get in the other C++ containers. So there’s no difference in runtime performance, but there are still the extra features we get in std::array.

Q4. Can we have a C++ std array of unknown size?

No, we can’t have a C++ std array of unknown size. 

Q5. Can we return an entire array in C++ as an argument to a function?

In C++, we cannot return an entire array as an argument to a function. However, by specifying the array's name without an index, we can return a pointer to an array.

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