在结构中返回多个值

Question

So I have this code but I have a couple things that need tweaking...

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

struct Student {
    string major;
    string name;
    int year;
};

string yearToString(Student s) {
    string stringYear;
    ostringstream convert;
    convert << s.year;
    stringYear = convert.str();
    cout << stringYear << endl; // this prints 3
    return stringYear; // this returns nothing
}

int main() {
    Student* students = new Student[2];

    students[0].major = "Computer Science";
    students[0].name = "Jermaine";
    students[0].year = 2;

    students[1].major = "Biology";
    students[1].name = "Kelsey";
    students[1].year = 3;

    yearToString(students[1]);

    delete[] students;
    return (0);
}

I was wondering why my return statement doesn't return the value 3? Also lets say I wanted to return multiple values of the struct. How would I return something like "Jermaine, Computer Science (2)". Can anyone point me in the right direction?

Answer 1

How do you know this returns nothing...

yearToString(students[1]);

... when you don't do anything with the return? Did you mean to assign it to something?

std::string yrStr = yearToString(students[1]);
std::cout << yrStr << std::endl; // prints 3

Answer 2

You don't use the returned value in any way. Changing the code to:

cout << yearToString(students[1]) << endl;

prints 3 as expected.

You seems to mix struct s, which represent data in memory, with formatted strings that are presented to the user. What you probably want to return seems to be the latter. In that case, all you need to do is format the struct:

ostringstream ostr;
ostr << s.name << ", " << s.major << " (" << s.year << ")";
return ostr.str();

Answer 3

To return multiple values from a structure, there are two primary choices:

1. Place into a struct, return the struct.
2. Pass a struct as a parameter, by reference, modify the values in the parameter.

First, define the structure:

struct Result
{
  std::string student_name;
  std::string course_name;
};

Returning the values:

Result My_Function()
{
  Result value;
  value.student_name = "Jermaine";
  value.course_name  = "Biology 101";
  return value;
}

Modifying a parameter:

void Sams_Function(Result& value)
{
  value.student_name = "Jermaine";
  value.course_name  = "Biology 101";
}

Many variables by reference:

Another method, not preferred, is to pass many variables by reference:

void Another_Function(std::string& student_name,
                      std::string& course_name)
{
      student_name = "Jermaine";
      course_name  = "Biology 101";
}