Returning multiple values from a C++ function

For returning two values I use a std::pair (usually typedef'd). You should look at boost::tuple (in C++11 and newer, there's std::tuple) for more than two return results.

With introduction of structured binding in C++ 17, returning std::tuple should probably become accepted standard.

Personally, I generally dislike return parameters for a number of reasons:

• it is not always obvious in the invocation which parameters are ins and which are outs
• you generally have to create a local variable to catch the result, while return values can be used inline (which may or may not be a good idea, but at least you have the option)
• it seems cleaner to me to have an "in door" and an "out door" to a function -- all the inputs go in here, all the outputs come out there
• I like to keep my argument lists as short as possible

I also have some reservations about the pair/tuple technique. Mainly, there is often no natural order to the return values. How is the reader of the code to know whether result.first is the quotient or the remainder? And the implementer could change the order, which would break existing code. This is especially insidious if the values are the same type so that no compiler error or warning would be generated. Actually, these arguments apply to return parameters as well.

Here's another code example, this one a bit less trivial:

pair<double,double> calculateResultingVelocity(double windSpeed, double windAzimuth,
                                               double planeAirspeed, double planeCourse);

pair<double,double> result = calculateResultingVelocity(25, 320, 280, 90);
cout << result.first << endl;
cout << result.second << endl;

Does this print groundspeed and course, or course and groundspeed? It's not obvious.

Compare to this:

struct Velocity {
    double speed;
    double azimuth;
};
Velocity calculateResultingVelocity(double windSpeed, double windAzimuth,
                                    double planeAirspeed, double planeCourse);

Velocity result = calculateResultingVelocity(25, 320, 280, 90);
cout << result.speed << endl;
cout << result.azimuth << endl;

I think this is clearer.

So I think my first choice, in general, is the struct technique. The pair/tuple idea is likely a great solution in certain cases. I'd like to avoid the return parameters when possible.

std::pair<int, int> divide(int dividend, int divisor)
{
   // :
   return std::make_pair(quotient, remainder);
}

std::pair<int, int> answer = divide(5,2);
 // answer.first == quotient
 // answer.second == remainder

std::pair is essentially your struct solution, but already defined for you, and ready to adapt to any two data types.

It's entirely dependent upon the actual function and the meaning of the multiple values, and their sizes:

• If they're related as in your fraction example, then I'd go with a struct or class instance.
• If they're not really related and can't be grouped into a class/struct then perhaps you should refactor your method into two.
• Depending upon the in-memory size of the values you're returning, you may want to return a pointer to a class instance or struct, or use reference parameters.

There are a bunch of ways to return multiple parameters. I'm going to be exhastive.

Use reference parameters:

void foo( int& result, int& other_result );

use pointer parameters:

void foo( int* result, int* other_result );

which has the advantage that you have to do a & at the call-site, possibly alerting people it is an out-parameter.

Write a template and use it:

template<class T>
struct out {
  std::function<void(T)> target;
  out(T* t):target([t](T&& in){ if (t) *t = std::move(in); }) {}
  out(std::optional<T>* t):target([t](T&& in){ if (t) t->emplace(std::move(in)); }) {}
  out(std::aligned_storage_t<sizeof(T), alignof(T)>* t):
    target([t](T&& in){ ::new( (void*)t ) T(std::move(in)); } ) {}
  template<class...Args> // TODO: SFINAE enable_if test
  void emplace(Args&&...args) {
    target( T(std::forward<Args>(args)...) );
  }
  template<class X> // TODO: SFINAE enable_if test
  void operator=(X&&x){ emplace(std::forward<X>(x)); }
  template<class...Args> // TODO: SFINAE enable_if test
  void operator()(Args...&&args){ emplace(std::forward<Args>(args)...); }
};

then we can do:

void foo( out<int> result, out<int> other_result )

and all is good. foo is no longer able to read any value passed in as a bonus.

Other ways of defining a spot you can put data can be used to construct out. A callback to emplace things somewhere, for example.

We can return a structure:

struct foo_r { int result; int other_result; };
foo_r foo();

whick works ok in every version of C++, and in c++17 this also permits:

auto&&[result, other_result]=foo();

at zero cost. Parameters can even not even be moved thanks to guaranteed elision.

We could return a std::tuple:

std::tuple<int, int> foo();

which has the downside that parameters are not named. This permits the c++17:

auto&&[result, other_result]=foo();

as well. Prior to c++17 we can instead do:

int result, other_result;
std::tie(result, other_result) = foo();

which is just a bit more awkward. Guaranteed elision doesn't work here, however.

Going into stranger territory (and this is after out<>!), we can use continuation passing style:

void foo( std::function<void(int result, int other_result)> );

and now callers do:

foo( [&](int result, int other_result) {
  /* code */
} );

a benefit of this style is you can return an arbitrary number of values (with uniform type) without having to manage memory:

void get_all_values( std::function<void(int)> value )

the value callback could be called 500 times when you get_all_values( [&](int value){} ).

For pure insanity, you could even use a continuation on the continuation.

void foo( std::function<void(int, std::function<void(int)>)> result );

whose use looks like:

foo( [&](int result, auto&& other){ other([&](int other){
  /* code */
}) });

which would permit many-one relationships between result and other.

Again with uniforn values, we can do this:

void foo( std::function< void(span<int>) > results )

here, we call the callback with a span of results. We can even do this repeatedly.

Using this, you can have a function that efficiently passes megabytes of data without doing any allocation off the stack.

void foo( std::function< void(span<int>) > results ) {
  int local_buffer[1024];
  std::size_t used = 0;
  auto send_data=[&]{
    if (!used) return;
    results({ local_buffer, used });
    used = 0;
  };
  auto add_datum=[&](int x){
    local_buffer[used] = x;
    ++used;
    if (used == 1024) send_data();
  };
  auto add_data=[&](gsl::span<int const> xs) {
    for (auto x:xs) add_datum(x);
  };
  for (int i = 0; i < 7+(1<<20); ++i) {
    add_datum(i);
  }
  send_data(); // any leftover
}

Now, std::function is a bit heavy for this, as we would be doing this in zero-overhead no-allocation environments. So we'd want a function_view that never allocates.

Another solution is:

std::function<void(std::function<void(int result, int other_result)>)> foo(int input);

where instead of taking the callback and invoking it, foo instead returns a function which takes the callback.

foo(7)([&](int result, int other_result){ /* code */ });

this breaks the output parameters from the input parameters by having separate brackets.

With variant and c++20 coroutines, you could make foo a generator of a variant of the return types (or just the return type). The syntax is not yet fixed, so I won't give examples.

In the world of signals and slots, a function that exposes a set of signals:

template<class...Args>
struct broadcaster;

broadcaster<int, int> foo();

allows you to create a foo that does work async and broadcasts the result when it is finished.

Down this line we have a variety of pipeline techniques, where a function doesn't do something but rather arranges for data to be connected in some way, and the doing is relatively independant.

foo( int_source )( int_dest1, int_dest2 );

then this code doesn't do anything until int_source has integers to provide it. When it does, int_dest1 and int_dest2 start recieving the results.

The OO solution for this is to create a ratio class. It wouldn't take any extra code (would save some), would be significantly cleaner/clearer, and would give you some extra refactorings letting you clean up code outside this class as well.

Actually I think someone recommended returning a structure, which is close enough but hides the intent that this needs to be a fully thought-out class with constructor and a few methods, in fact, the "method" that you originally mentioned (as returning the pair) should most likely be a member of this class returning an instance of itself.

I know your example was just an "Example", but the fact is that unless your function is doing way more than any function should be doing, if you want it to return multiple values you are almost certainly missing an object.

Don't be afraid to create these tiny classes to do little pieces of work--that's the magic of OO--you end up breaking it down until every method is very small and simple and every class small and understandable.

Another thing that should have been an indicator that something was wrong: in OO you have essentially no data--OO isn't about passing around data, a class needs to manage and manipulate it's own data internally, any data passing (including accessors) is a sign that you may need to rethink something..

There is precedent for returning structures in the C (and hence C++) standard with the div, ldiv (and, in C99, lldiv) functions from <stdlib.h> (or <cstdlib>).

The 'mix of return value and return parameters' is usually the least clean.

Having a function return a status and return data via return parameters is sensible in C; it is less obviously sensible in C++ where you could use exceptions to relay failure information instead.

If there are more than two return values, then a structure-like mechanism is probably best.

Use a struct or a class for the return value. Using std::pair may work for now, but

1. it's inflexible if you decide later you want more info returned;
2. it's not very clear from the function's declaration in the header what is being returned and in what order.

Returning a structure with self-documenting member variable names will likely be less bug-prone for anyone using your function. Putting my coworker hat on for a moment, your divide_result structure is easy for me, a potential user of your function, to immediately understand after 2 seconds. Messing around with ouput parameters or mysterious pairs and tuples would take more time to read through and may be used incorrectly. And most likely even after using the function a few times I still won't remember the correct order of the arguments.

If your function returns a value via reference, the compiler cannot store it in a register when calling other functions because, theoretically, the first function can save the address of the variable passed to it in a globally accessible variable, and any subsecuently called functions may change it, so the compiler will have (1) save the value from registers back to memory before calling other functions and (2) re-read it when it is needed from the memory again after any of such calls.

If you return by reference, optimization of your program will suffer

C++17, using std::make_tuple, structured binding and as much auto as possible:

#include <tuple>

#include <string>
#include <cstring>

auto func() {
    // ...
    return std::make_tuple(1, 2.2, std::string("str"), "cstr");
}

int main() {
    auto [i, f, s, cs] = func();
    return i + f + s.length() + strlen(cs);
}

With -O1 this optimizes out completely: https://godbolt.org/z/133rT9Pcq
-O3 needed only to optimize out std::string: https://godbolt.org/z/Mqbez73Kf

And here: https://godbolt.org/z/WWKvE3osv you can see GCC storing all the returned values packed together in a single chunk of memory (rdi+N), POD-style, proving there is no performance penalty.

I tend to use out-vals in functions like this, because I stick to the paradigm of a function returning success/error codes and I like to keep things uniform.

Alternatives include arrays, generators, and inversion of control, but none is appropriate here.

Some (e.g. Microsoft in historical Win32) tend to use reference parameters for simplicity, because it's clear who allocates and how it will look on the stack, reduces the proliferation of structures, and allows a separate return value for success.

"Pure" programmers prefer the struct, assuming it is the function value (as is the case here), rather than something that's touched incidentally by the function. If you had a more complicated procedure, or something with state, you'd probably use references (assuming you have a reason for not using a class).

I'd say there is no preferred method, it all depends on what you're going to do with the response. If the results are going to be used together in further processing then structures make sense, if not I'd tend to pass then as individual references unless the function was going to be used in a composite statement:

x = divide( x, y, z ) + divide( a, b, c );

I often choose to pass 'out structures' by reference in the parameter list rather than having the pass by copy overhead of returning a new structure (but this is sweating the small stuff).

void divide(int dividend, int divisor, Answer &ans)

Are out parameters confusing? A parameter sent as reference suggests the value is going to change (as opposed to a const reference). Sensible naming also removes confusion.

rather than returning multiple values,just return one of them and make a reference of others in the required function for eg:

int divide(int a,int b,int quo,int &rem)

Here is the link to the "core guidelines" (by Bjarne Stroustrup and Herb Sutter) on this topic.

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-out-multi

Partial Quote:

F.21: To return multiple “out” values, prefer returning a struct or tuple

Reason A return value is self-documenting as an “output-only” value. Note that C++ does have multiple return values, by convention of using a tuple (including pair), possibly with the extra convenience of tie or structured bindings (C++17) at the call site. Prefer using a named struct where there are semantics to the returned value. Otherwise, a nameless tuple is useful in generic code.

Boost tuple would be my preferred choice for a generalized system of returning more than one value from a function.

Possible example:

include "boost/tuple/tuple.hpp"

tuple <int,int> divide( int dividend,int divisor ) 

{
  return make_tuple(dividend / divisor,dividend % divisor )
}

I would just do it by reference if it's only a few return values but for more complex types you can also just do it like this :

static struct SomeReturnType {int a,b,c; string str;} SomeFunction()
{
  return {1,2,3,string("hello world")}; // make sure you return values in the right order!
}

use "static" to limit the scope of the return type to this compilation unit if it's only meant to be a temporary return type.

 SomeReturnType st = SomeFunction();
 cout << "a "   << st.a << endl;
 cout << "b "   << st.b << endl;
 cout << "c "   << st.c << endl;
 cout << "str " << st.str << endl;

This is definitely not the prettiest way to do it but it will work.

Quick answer:

#include <iostream>
using namespace std;

// different values of [operate] can return different number.
int yourFunction(int a, int b, int operate)
{
    a = 1;
    b = 2;

    if (operate== 1)
    {
        return a;
    }
    else
    {
        return b;
    }
}

int main()
{
    int a, b;

    a = yourFunction(a, b, 1); // get return 1
    b = yourFunction(a, b, 2); // get return 2

    return 0;
}