15.7. Fixed-size sequences with std::array¶

std::array is declared in <array>. Its element type and size are both part of its type: std::array<int, 3> and std::array<int, 4> are different types. Use it when the number of elements is known at compile time.

Example c192_arrays_readings

 1#include <array>
 2#include <iostream>
 3
 4int main() {
 5    std::array<int, 4> readings{18, 21, 19, 22};
 6    int total = 0;
 7    for (int reading : readings) {
 8        total += reading;
 9    }
10    std::cout << readings.size() << " readings, total " << total << '\n';
11    readings.at(0) = 20;
12    std::cout << readings.front() << '\n';
13}

List initialization supplies the elements in order. std::array<int, 4> a{}; initializes all four integers to zero. In contrast, a local declaration std::array<int, 4> a; does not initialize those integers; do not read them before assigning values.

size() returns an unsigned size type. If you need a position, use std::size_t from <cstddef>; if you only need the elements, prefer a range-based for loop. at() checks its index and throws std::out_of_range if it is invalid. operator[] does not perform this check in C++20. Neither front() nor back() may be used on an empty array.

An array owns its elements and can be copied or assigned. Its size never changes: it has no push_back or resize member. A built-in C array lacks many of these conveniences, so we use std::array for fixed-size sequences.

Nested arrays can describe a rectangular table. The outer array contains rows; each row contains columns. Access table[row][column], using two pairs of brackets, not table[row, column].

Example c192_arrays_table

 1#include <array>
 2#include <iostream>
 3
 4int main() {
 5    std::array<std::array<int, 3>, 2> table{{{1, 2, 3}, {4, 5, 6}}};
 6    for (const auto& row : table) {
 7        for (int value : row) {
 8            std::cout << value << ' ';
 9        }
10        std::cout << '\n';
11    }
12    std::cout << table.at(1).at(2) << '\n';
13}

The dimensions here are two rows and three columns. Each at() checks one dimension. The elements in an array of arrays have fixed rectangular shape.

We want to open a file and parse its data into our program. What library do we need to include?

iostream

This library deals with communication through the standard input and output.

sstream

This library is used to manipulate string objects as if they were streams.

fstream

This library is used to manipulate files using streams.

iomanip

This library is used to modify internal flags and formatting options.

In table[9][17], the row index is and the column index is .