Project 5.09 Section 5 ⚡ Embedded Relevance: Core Pass-by-Reference Out Parameters Multiple Returns Structured Binding C++17

5.09 In-Out Parameters, Multiple Return Values & Struct Results vs std::tuple

Executive Summary: Exploring functions that return multiple values via pass-by-reference out-parameters. We compare legacy out-parameters with modern C++17 structured bindings and small return structs.

💻 1. Annotated Source Code

#include <iostream>
using namespace std;

void threeTimesN(int input, int& output);

int main() {

	int myInput = 50;
	int myOutput;

	threeTimesN(myInput, myOutput);

	cout << "After threeTimesN, myOuput is " << myOutput << endl;

	return 0;
}

void threeTimesN(int input, int& output) {
	output = input * 3;
}

📐 2. Architecture & UML Class Model

📐 Parameter Passing Challenge & Out-Parameter Design

+ Public - Private # Protected

<<compilation-unit>> ParameterChallengeUnit Out Parameters

Attributes / Data Members

(none / stateless)

Operations / Methods

+computeStats(a: int, b: int, sum: int&, product: int&) : void

📚 3. Core C++ Concepts Deep-Dive

1. Out-Parameters via References

Functions requiring multiple output values historically passed references or pointers as out-parameters (void getCoordinates(int& x, int& y)).

2. Modern Alternative: Value Structs & Structured Bindings

In C++17, returning a small struct by value is optimized via Return Value Optimization (RVO), enabling clean structured binding syntax: auto [x, y] = getCoordinates();.

⚡ 4. Embedded Systems & Hardware Reality

1. RVO and Register Packing

Under the ARM AAPCS, small return structs containing two 32-bit integers are returned packed in registers R0 and R1 with 0 RAM overhead.

💡 5. Production-Ready Embedded Refactoring

Modern struct return with structured binding support:

💡 Production-Ready Refactor

#include <cstdint>

struct Coordinate2D {
    int32_t x;
    int32_t y;
};

// Returned packed in registers R0 and R1 (0 stack traffic!)
constexpr Coordinate2D readGpsPosition() noexcept {
    return Coordinate2D{12345, 67890};
}

📝 Knowledge Verification Quiz

Test your understanding of the C++ concepts and embedded microcontroller trade-offs covered in this guide. Click any option for instant feedback.

Q1. What feature introduced in C++17 allows cleanly unpacking members from a returned struct (auto [x, y] = getPos())?

A Structured Bindings

B Lambda Expressions

C Concepts

D Dynamic Casting

Detailed Explanation: Structured bindings (C++17) allow directly unpacking struct members into local variables.

Q2. How are small 2-word structs returned by value on ARM Cortex-M processors?

A Returned directly in hardware registers R0 and R1 without touching memory

B Allocated on the heap

C Stored in Flash memory

D Sent over the UART serial bus

Detailed Explanation: Under AAPCS, return values up to 8 bytes (two 32-bit words) are passed back directly in CPU registers R0 and R1.

Q3. Why is returning a struct by value often cleaner than using multiple non-const reference out-parameters?

A It makes function inputs and outputs explicit at the call site and eliminates pointer aliasing issues

B It allows functions to have duplicate names

C It disables compiler warnings

D It makes the function run in background threads

Detailed Explanation: Returning structs keeps data flow clear and functional, avoiding side-effect bugs and compiler aliasing penalties.

Q4. What is Return Value Optimization (RVO)?

A A compiler optimization that constructs a returned object directly into the caller's target storage, eliminating copy and move operations

B A technique to compress return values in Flash

C A method for checking return codes at runtime

D A compiler flag that deletes unused functions

Detailed Explanation: RVO constructs the return value directly in the destination memory allocated by the caller, achieving zero copy overhead.