Return the addition of variadic parameter pack

Question

Let's say I have a function sum that takes a variadic parameter pack.
This function needs to ADD UP all of the parameters of the parameter pack using the operator +.

NOTE: It CANNOT use the operator += in any way and can use only operator + (as the program CANNOT assume that all parameters have operator+= overloaded).

Then, function sum needs to return the whole "sum" or accumulative add-up.

Here is the basic structure of what it would look like:

template <class... Args>
auto sum(Args... args)
{
    // ... The code
}

NOTE: All arguments may not be of the same type AND you can assume that a corresponding overload of the operator+ exists for all types of the parameter pack.

It may be helpful to know that I am using C++ 20.

Answer 1

Use C++17's fold expressions, https://en.cppreference.com/w/cpp/language/fold. You don't need to specify the return type in most cases.

#include<cassert>

template <class... Args>
auto sum(Args... args) {  // or right_fold_plus
    return (args + ...);
}

int main() {
    auto s = sum(1, 2, 3);
    assert( s == 1 + 2 + 3 );
}

https://godbolt.org/z/1Yh118ETb

NOTE: It is pretty much an accident of the language that one needs a function to do this, which, unfortunately, opens the opportunity to give the "wrong name" to a function (in the sense that it can do something that the name doesn't imply, like concatenating strings). In that sense, sum is not a good name because it could also be called concatenate. A more neutral name, without semantic implications, would be to call it right_fold_plus.

... or simply not give it a name it at all [](auto... xs) {return (xs + ...);}(1, 2, 3) (see https://godbolt.org/z/7x9PzrzY1)