How to return a struct from a function in C++?

Question

I've tried on a few different forums and can't seem to get a straight answer, how can I make this function return the struct? If I try 'return newStudent;' I get the error 'No suitable user-defined conversion from studentType to studentType exists.'

// Input function
studentType newStudent()
{   
    struct studentType
    {
        string studentID;
        string firstName;
        string lastName;
        string subjectName;
        string courseGrade;

        int arrayMarks[4];

        double avgMarks;

    } newStudent;

    cout << "\nPlease enter student information:\n";

    cout << "\nFirst Name: ";
    cin >> newStudent.firstName;

    cout << "\nLast Name: ";
    cin >> newStudent.lastName;

    cout << "\nStudent ID: ";
    cin >> newStudent.studentID;

    cout << "\nSubject Name: ";
    cin >> newStudent.subjectName;

    for (int i = 0; i < NO_OF_TEST; i++)
    {   cout << "\nTest " << i+1 << " mark: ";
        cin >> newStudent.arrayMarks[i];
    }

    newStudent.avgMarks = calculate_avg(newStudent.arrayMarks,NO_OF_TEST );
    newStudent.courseGrade = calculate_grade (newStudent.avgMarks);

}

You seem to have declared two versions of `studentType` define: one outside the function and one inside the function. You should have only one definition, the one outside the function. — Dietmar Kühl, Oct 06 '13 at 03:14
Alternatively, you can allocate it on the heap and return a pointer of type studentType. — Mike G, Oct 06 '13 at 03:16

score 45 · Answer 1 · edited Apr 01 '16 at 12:39

45

Here is an edited version of your code which is based on ISO C++ and which works well with G++:

#include <string.h>
#include <iostream>
using namespace std;

#define NO_OF_TEST 1

struct studentType {
    string studentID;
    string firstName;
    string lastName;
    string subjectName;
    string courseGrade;
    int arrayMarks[4];
    double avgMarks;
};

studentType input() {
    studentType newStudent;
    cout << "\nPlease enter student information:\n";

    cout << "\nFirst Name: ";
    cin >> newStudent.firstName;

    cout << "\nLast Name: ";
    cin >> newStudent.lastName;

    cout << "\nStudent ID: ";
    cin >> newStudent.studentID;

    cout << "\nSubject Name: ";
    cin >> newStudent.subjectName;

    for (int i = 0; i < NO_OF_TEST; i++) {
        cout << "\nTest " << i+1 << " mark: ";
        cin >> newStudent.arrayMarks[i];
    }

    return newStudent;
}

int main() {
    studentType s;
    s = input();

    cout <<"\n========"<< endl << "Collected the details of "
        << s.firstName << endl;

    return 0;
}

edited Apr 01 '16 at 12:39

pevik

4,523
3
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44

answered Oct 06 '13 at 05:21

Nandakumar Edamana

770
1
4
10

2

Isn't it more efficient to prevent the copy constructor by using a pointer? – Javier Cabero Feb 08 '19 at 10:31
@JavierCabero I think there will be a copy-elision automatically... so no copy at all! But if you really want to manage that, you can implement some lvalue move on the struct object. – Sandburg Apr 05 '19 at 14:56
2

Isn't it bad to return a variable that has been allocated on the stack? – Little Helper Mar 17 '20 at 21:22
1

@LittleHelper: That would only be a problem if you return by reference. – rkgghz Jul 07 '20 at 09:25

score 24 · Accepted Answer · answered Oct 06 '13 at 03:13

24

You have a scope problem. Define the struct before the function, not inside it.

answered Oct 06 '13 at 03:13

Fred Larson

60,987
18
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174

Rehan · Answer 3 · 2023-03-12T02:06:15.527

16

You can now (C++14) return a locally-defined (i.e. defined inside the function) struct as follows:

auto f()
{
    struct S
    {
      int a;
      double b;
    } s;
    s.a = 42;
    s.b = 42.0;
    return s;
}

auto x = f();
a = x.a;
b = x.b;

edited Mar 12 '23 at 02:06

answered Feb 18 '20 at 21:19

Rehan

1,299
2
16
19

score 8 · Answer 4 · answered Oct 06 '13 at 03:15

8

studentType newStudent() // studentType doesn't exist here
{   
    struct studentType // it only exists within the function
    {
        string studentID;
        string firstName;
        string lastName;
        string subjectName;
        string courseGrade;

        int arrayMarks[4];

        double avgMarks;

    } newStudent;
...

Move it outside the function:

struct studentType
{
    string studentID;
    string firstName;
    string lastName;
    string subjectName;
    string courseGrade;

    int arrayMarks[4];

    double avgMarks;

};

studentType newStudent()
{
    studentType newStudent
    ...
    return newStudent;
}

answered Oct 06 '13 at 03:15

Zac Howland

15,777
1
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42

as you are not using `new` operator to create the structure instance, will it be deleted automatically after you call `newStudent()` – Ramesh-X Feb 08 '18 at 10:28
2

@Ramesh-X No, because `new` creates a *pointer* to an instance created on the heap. If you return the structure directly, it just "copies" (a sufficiently intelligent compiler may just use the location that the caller provided for the returned struct) the bits of the structure into the lvalue that the result of the function call was assigned to in the caller. The problem with returning a local structure arises when you return a "pointer" (the pointer may be a reference, which is not actually required to be implemented using pointers) to the local structure. – cuddlebugCuller Sep 24 '20 at 21:53

score -1 · Answer 5 · answered Oct 06 '13 at 03:39

-1

As pointed out by others, define studentType outside the function. One more thing, even if you do that, do not create a local studentType instance inside the function. The instance is on the function stack and will not be available when you try to return it. One thing you can however do is create studentType dynamically and return the pointer to it outside the function.

answered Oct 06 '13 at 03:39

mkny

13
4

7

If he creates an automatic `studentType` and returns it by copy, the copy constructor will be called (a default one is created if you don't create one), so he will not have any issues. Declaring the structure inside the function is his primary problem - and there is no need to declare it dynamically at present. – Zac Howland Oct 06 '13 at 04:24
@ZacHowland AIUI, the default method for returning something from a function is by *move*, not by copy. The callee doesn't need it anymore, after all. – cuddlebugCuller Sep 24 '20 at 21:56
2

@cuddlebugCuller When the comment was made, that was not the case :) The move semantics have been added and updated a couple times since 2013. – Zac Howland Oct 26 '20 at 04:06

How to return a struct from a function in C++?

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