std::array

What problem does this section solve?

C-style arrays (int arr[5]) have many drawbacks: they cannot be assigned directly, they decay into pointers when passed to functions (losing size information), and there is no bounds checking. Dynamic allocation (std::vector), however, is powerful but involves dynamically allocating memory, which comes with a small overhead.

std::array addresses the need for a compile-time sized, stack-allocated array that offers the same performance as a C array but with an STL container interface (including size(), iterators, and assignability), providing a safe alternative.

What is this feature?

std::array<T, N> is a container template in the STL that encapsulates a fixed-size array. Its size is determined at compile time and does not dynamically allocate memory (it resides on the stack), wrapping a C array while providing the standard interface of an STL container.

C++ standard version

C++11

Required header files

#include <array>

运行结果： 此代码块不含 main 函数，是语法、接口或分文件示例，不能独立运行，因此没有终端输出。

Basic Syntax

std::array<元素类型, 大小> 变量名;            // 默认初始化（值未定义）
std::array<元素类型, 大小> 变量名 = {};        // 全部初始化为零
std::array<元素类型, 大小> 变量名 = {v1, v2, ...}; // 列表初始化

运行结果： 此代码块不含 main 函数，是语法、接口或分文件示例，不能独立运行，因此没有终端输出。

Common Usage

Example code

Example 1: Create an array and access elements

#include <iostream>
#include <array>

using namespace std;

int main()
{
    // 创建一个包含 5 个 int 的 array，并初始化
    std::array<int, 5> arr = {10, 20, 30, 40, 50};

    // 用下标访问
    std::cout << "arr[0] = " << arr[0] << "\n";
    std::cout << "arr[4] = " << arr[4] << "\n";

    // 用 size() 获取大小
    std::cout << "size = " << arr.size() << "\n";

    return 0;
}

Results：

arr[0] = 10
arr[4] = 50
size = 5

Example 2: Based on Example 1, using at() for safe access and fill() for filling

#include <iostream>
#include <array>

using namespace std;

int main()
{
    std::array<int, 5> arr = {10, 20, 30, 40, 50};

    // 用 at() 安全访问
    std::cout << "arr.at(2) = " << arr.at(2) << "\n";

    // 用 fill() 将所有元素设为同一个值
    arr.fill(99);

    std::cout << "after fill: ";
    for (int n : arr)
    {
        std::cout << n << " ";
    }
    std::cout << "\n";

    return 0;
}

Results：

arr.at(2) = 30
after fill: 99 99 99 99 99 

Example 3: Building on Example 2, comparing C arrays and std::array parameter passing

#include <iostream>
#include <array>

using namespace std;

// C 风格：数组退化为指针，丢失大小信息
void print_c_array(int* arr, int size)
{
    std::cout << "C array: ";
    for (int i = 0; i < size; ++i)
    {
        std::cout << arr[i] << " ";
    }
    std::cout << "\n";
}

// std::array：大小信息不丢失
void print_std_array(const std::array<int, 5>& arr)
{
    std::cout << "std::array: ";
    for (int n : arr)
    {
        std::cout << n << " ";
    }
    std::cout << "\n";
    std::cout << "size from inside function = " << arr.size() << "\n";
}

int main()
{
    std::array<int, 5> arr = {1, 2, 3, 4, 5};

    // C 风格：需要额外传大小
    print_c_array(arr.data(), arr.size());

    // std::array：自带大小信息
    print_std_array(arr);

    return 0;
}

Results：

C array: 1 2 3 4 5 
std::array: 1 2 3 4 5 
size from inside function = 5

runtime results

See the "running results" for each example above.

Key syntax explanation in the example

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The differences between array and vector become most evident in the following scenarios:

• When fixed-size storage is needed: If you know the exact number of elements at compile time and this size won't change, array is more efficient.
• When dynamic resizing is required: If the number of elements may change during runtime, vector is the clear choice.
• In performance-sensitive contexts: array avoids the overhead of heap allocation and dynamic resizing, making it faster in tight loops or embedded systems.
• When passing data to low-level APIs: array provides a contiguous, fixed-size block of memory that aligns well with C-style functions.

If just storing 5 integers, array and vector seem to both work. However, within a project, the differences mainly lie in "whether the size is fixed" and "whether dynamic expansion is needed":

array is more like "a fixed-size array with an STL interface," while vector is more like "a resizable array." Don't use array everywhere just because it's more lightweight: whenever the number of elements isn't known until runtime, you should use vector.

Example 4: Fixed three-axis data is better suited for array.

#include <array>
#include <iostream>

using namespace std;

// std::array 是固定长度数组，长度在编译期就确定。
double norm3(const std::array<double, 3>& v)
{
    return v[0] * v[0] + v[1] * v[1] + v[2] * v[2];
}

int main()
{
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。
    std::array<double, 3> accel = {0.1, 0.2, 9.8};

    std::cout << "accel size = " << accel.size() << "\n";
    std::cout << "squared norm = " << norm3(accel) << "\n";

    return 0;
}

Results：

accel size = 3
squared norm = 96.09

In this example, the three-axis data must be exactly 3 values. Using std::array<double, 3> allows this constraint to be written into the type; if std::array<double, 2> is mistakenly passed, it will result in a compile-time error.

Common Errors

Error 1: Specifying array size with a variable

int n = 5;
std::array<int, n> arr;  // ❌ 编译错误！n 必须是编译期常量

运行结果： 此代码块不含 main 函数，是语法、接口或分文件示例，不能独立运行，因此没有终端输出。

The correct approach: use the constexpr constant, or alternatively switch to std::vector.

Error 2: Accessing before initialization

std::array<int, 5> arr;   // 值未定义（栈上的垃圾值）
std::cout << arr[0];      // ❌ 未定义行为

运行结果： 此代码块不含 main 函数，是语法、接口或分文件示例，不能独立运行，因此没有终端输出。

Correct approach: Initialize with {} or use fill().

Error 3: Assigning between arrays of different sizes

std::array<int, 3> a = {1, 2, 3};
std::array<int, 5> b = a;  // ❌ 编译错误！类型不同

运行结果： 此代码块不含 main 函数，是语法、接口或分文件示例，不能独立运行，因此没有终端输出。

The correct approach: std::array<int,3> and std::array<int,5> are of different types and cannot be assigned to each other.

使用建议

• 明确目标：在开始前确定您的具体需求，以便选择最合适的工具或教程。
• 充分利用资源：参考官方文档、教程和博客，这些资料能帮助您快速上手并解决问题。
• 实践应用：通过动手操作项目或编写代码来巩固学习成果，提升实际操作能力。
• 问题解决：遇到困难时，查阅参考资料或寻求社区支持，逐步培养独立解决问题的能力。
• 分享经验：完成项目后，可以撰写文章或博客分享心得，帮助其他学习者。

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1. For sizes known at compile time, use an array：If the size is known at compile time, using std::array is more lightweight than std::vector (without heap allocation).
2. Replacing C Arrays: Use std::array whenever int arr[N] can be used.
3. Note the size when passing arguments: The size of std::array<T, N> is part of its type, and the function signature must specify N.
4. Assignable: Unlike C arrays, std::array can be assigned as a whole using = (of the same type).

Summary

• std::array<T, N> encapsulates a fixed-size array, with its size determined at compile time.
• Includes the standard interface of STL containers (size(), at(), iterators, etc.).
• Yes, you can assign the entire array, and when passing it to a function, it won’t decay into a pointer, so the size information is retained.
• Suitable for scenarios where the size is known at compile time, more lightweight than vector.