Structured bindings

Learn multi-variable assignment with structured bindings.

Structured bindings

Structured bindings (C++17) bind the members of a struct, pair or tuple to separate variables in one line. The syntax is auto [a, b] = p;. Here a and b become local names for p.first and p.second.

This is more readable than p.first and p.second. When a function returns multiple values via std::pair or std::tuple, the caller unpacks the result easily with a binding.

Multiple returns with std::pair

A function returns two values with std::pair<T1, T2>. The caller unpacks with auto [a, b] = func();. Example: std::pair<bool, int> find(int x) returns whether it found and the value.

Structured bindings are cleaner than std::get<0> and std::get<1>. The compiler creates one variable per member with no run-time cost. The same syntax works for std::array and plain structs.

Exercise Bind the min_max function

Write std::pair<int, int> min_max(int a, int b): put the smaller value in first, the larger in second. In main, bind the result with auto [mn, mx] = min_max(7, 3); and print mn via std::cout.

Show example solution

#include <iostream>
#include <utility>

std::pair<int, int> min_max(int a, int b) {
    return {std::min(a, b), std::max(a, b)};
}

int main() {
    auto [mn, mx] = min_max(7, 3);
    std::cout << mn;
    return 0;
}

Test yourself

Complete the binding syntax

auto ____ = make_pair(3, 7);

Show answer

auto [a, b] = make_pair(3, 7);

Which line is a valid structured binding?
- auto (a, b) = p;
- auto [a, b] = p;
- auto {a, b} = p;
- auto <a, b> = p;
Show answer

auto [a, b] = p; — Square brackets are used for structured bindings.
Match the syntax to its purpose

auto [a, b] = p

Bind pair members
auto [a, b, c] = t

Bind tuple members
auto& [a, b] = p

Bind by reference
auto [a, b]{a, b}

Not a template

Next lesson: if constexpr

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