Arrays, maps, sets, and container choice

Container choice should follow how the data is used. Start with std::vector, then switch when another container matches the problem better.

std::array

std::array<T, N> stores a fixed number of elements:

std::array<int, 3> rgb{255, 128, 0};

Use it when the size is known at compile time and should not change.

Prefer std::array over raw arrays in beginner and intermediate code because it knows its size and works better with standard algorithms.

std::map

std::map<Key, Value> stores key-value pairs ordered by key:

std::map<std::string, int> counts;
counts["error"] += 1;

Use it when ordered iteration matters or when stable ordering is useful.

std::unordered_map

std::unordered_map<Key, Value> stores key-value pairs in a hash table:

std::unordered_map<std::string, int> counts;
counts["error"] += 1;

Use it for average fast lookup when ordering does not matter.

std::set

std::set<T> stores unique ordered values:

std::set<std::string> tags;
tags.insert("network");
tags.insert("storage");

Use a set when uniqueness is central and ordered iteration matters. If order does not matter, std::unordered_set may fit.

Choosing simply

Use this starting point:

• std::vector for sequences
• std::string for owned text
• std::array for fixed-size sequences
• std::unordered_map for key lookup without order
• std::map for key lookup with order
• std::set when unique sorted values are the main point

Do not choose a container by habit alone. Ask what operations dominate: iteration, lookup, insertion, removal, ordering, or memory locality.

Avoid raw arrays by default

Raw arrays appear in older C++ and in low-level interfaces:

int values[3]{1, 2, 3};

They are not the best default for modern code. Prefer std::array or std::vector unless you have a specific reason.

What to carry forward

• std::vector remains the default sequence container
• fixed-size data fits std::array
• lookup data often fits map or unordered map
• sets model uniqueness
• container choice depends on access pattern and constraints
• raw arrays are low-level tools, not beginner defaults

Next, you will learn iterators, algorithms, and invalidation.