Why do we use functions that return a data structure in C++? [duplicate]

Question

I have been learning C++ and came across a function, but the return type was a vector.

Here is the code:

vector<Name> inputNames() {
    ifstream fin("names.txt");
    string word;
    vector<Name> namelist;

    while (!fin.eof()) {
        Name name;
        fin >> name.first_name;
        fin >> name.last_name;
        namelist.push_back(name);
    }

    return namelist;
}

name is part of a struct defined as:

struct Name {
    string first_name;
    string last_name;

    bool operator<(const Name& d) const {
        return last_name > d.last_name;
    }

    void display() {
        cout << first_name << " " << last_name << endl;
    }
};

What is the purpose of using vector< Name>inputName()? What is it doing?

And why can I just not create a void function and pass a vector through it?

I.e.:

void input(vector<Name>&v){
    ifstream fin("names.txt");
    string word;

    while (!fin.eof()) {
        Name name;
        fin >> name.first_name;
        fin >> name.last_name;
        v.push_back(name);
    }
}

Answer 1

Your question is basically: Do I return by value or do I use an output argument?

The general consensus in the community is to return by value, especially from C++17 on with guaranteed copy elision. Although, I also recommend it C++11 onwards. If you use an older version, please upgrade.

We consider the first snippet more readable and understandable and even more performant.

From a callers perspective:

std::vector<Name> names = inputNames();

It's clear that inputNames returns you some values without changing the existing state of the program, assuming you don't use global variables (which you actually do with cin).

The second code would be called the following way:

std::vector<Name> names;
 // Other code
inputNames(names);

This raises a lot of questions:

• does inputNames use the names as input or does it extend it?
• if there are values in names, what does the function do with it?
• does the function have a return value to indicate success?

It used to be good practice when computers were slow and compilers had troubles optimizing, though, at this point, don't use it for output arguments.

When do you use the last style: if you want an in-out argument. In this case, if you intend to append, the vector already had data, and that actually makes sense.