Why Is It Ok to Return a 'Vector' from a Function

Why is it OK to return a 'vector' from a function?

Can we guarantee it will not die?

As long there is no reference returned, it's perfectly fine to do so. words will be moved to the variable receiving the result.

The local variable will go out of scope. after it was moved (or copied).

Efficient way to return a std::vector in c++

In C++11, this is the preferred way:

std::vector<X> f();

That is, return by value.

With C++11, std::vector has move-semantics, which means the local vector declared in your function will be moved on return and in some cases even the move can be elided by the compiler.

In C++, is it still bad practice to return a vector from a function?

Dave Abrahams has a pretty comprehensive analysis of the speed of passing/returning values.

Short answer, if you need to return a value then return a value. Don't use output references because the compiler does it anyway. Of course there are caveats, so you should read that article.

Return vector from function without it being destroyed

You can create anything on heap with new. You shouldn't give out from the function the references to your stack objects, as they will be destroyed as soon as the function finishes.

If you prefer your function to return the vector by value, be sure that the objects inside the vector implement copy constructor (and perhaps assignment operator, too, not sure about that). Having that, please do not forget about the Rule of Three.

returning vector of arrays from function in c++

Try with Standard Library arrays:

#include <iostream>
#include <vector>
#include <array>
using namespace std;

void process(vector<array<int, 3>> &vec)
{
    //vector<unsigned long long int*> vec;
    array<int, 3> a{1, 2, 3};
    array<int, 3> b{4, 5, 6};
    vec.push_back(a);
    vec.push_back(b);

    for (int i = 0; i < vec.size(); i++)
    {
        for (int j = 0; j < 3; j++)
        {
            cout << vec[i][j] << " ";
        }
        cout << "\n";
    }
    cout << "\n";

    //return vec;
}
int main()
{
    // your code goes here
    vector<array<int, 3>> vec;
    array<int, 3> c{1, 7, 8};
    vec.push_back(c);
    process(vec);
    for (int i = 0; i < vec.size(); i++)
    {
        for (int j = 0; j < 3; j++)
        {
            cout << vec[i][j] << " ";
        }
        cout << "\n";
    }
    cout << "\n";
    return 0;
}

C++ edit function, return vector

The way your methods are written, you cannot chain addContact() with another addContact() (or any other CEFaceMask method).

Either change the way you are calling addContact() the second time:

 test.addContact ( CContact ( CTimeStamp ( 2021, 1, 12, 12, 40, 10 ), 123456789, 111222333 ) );
 test.addContact ( CContact ( CTimeStamp ( 2021, 2, 5, 15, 30, 28 ), 999888777, 555000222 ) );

or change the signature of addContact() so that it can chain calls:

CEFaceMask& CEFaceMask::addContact(const CContact &c)
{
  m_Db.push_back(c);
  return *this;
}

return a vector vs use a parameter for the vector to return it

In theory, yes it's copied. In reality, no, most modern compilers take advantage of return value optimization.

So you can write code that acts semantically correct. If you want a function that modifies or inspects a value, you take it in by reference. Your code does not do that, it creates a new value not dependent upon anything else, so return by value.

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