06_std_array.md

06 — std::array<T, N>

Goal

Learn the basics of std::array: fixed size, STL integration, and useful APIs like .size(), .at(), and .data().

Concepts

• std::array<T, N> is a fixed-size container (like a C array), but with a container interface.
• .size() returns N.
• .at(i) checks bounds and throws std::out_of_range if i is invalid.
• .data() returns T* to the first element (useful to interoperate with C-style APIs).
• .fill(x) fills all elements with x.

Code explained (step by step)

File: 06_std_array.cpp

1. Create std::array<int,5> a{...}.
2. Show .size() and access via operator[].
3. Demonstrate .at() inside try/catch (bounds checking).
4. Use .data() to show the memory is contiguous.
5. Call a.fill(7) and print with range-based for.

Common mistakes

• Using operator[] expecting bounds checking (there isn’t; it’s like a C array).
• Storing a pointer from .data() and later using it incorrectly (with std::array it won’t be invalidated by reallocation, but still watch lifetime/scope).
• Thinking std::array is “dynamic” (it isn’t; the size is part of the type).

Exercises

1. Write a function that takes std::array<int, N> and returns the largest element.
2. Compare a[999] (UB) with a.at(999) (exception).
3. Convert a C array to std::array manually and compare usage.

How to build and run

g++ -std=c++20 -O2 -Wall -Wextra -pedantic -g 06_std_array.cpp -o app
./app