std::vector Struct Student example - c++

Question

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
struct StudentDataTypes
{
std::string name{};
int grade{};
};
int main()
{
//Ask for Class_Size
int Class_Size{};
std::cout << "How big is the class?" << '\n';
std::cin >> Class_Size;
//Syntax format
//std::vector<T> array(size);
//Intialize a vector for the students called Vector_Student
//T links to the struct StudentDataTypes
//size is the Class_Size.
std::vector<StudentDataTypes> Vector_Student(Class_Size);
//Print Class Size
std::cout << "There are " << Class_Size << " students." << '\n';
//Get the Userinputs for the Class
for (int i = 0; i < Class_Size; ++i)
{
std::cout << "Please input the name of Student #" << i + 1 << '\n';
std::cin >> Vector_Student[i].name;
std::cout << "Please input the grade of Student #" << i + 1 << '\n';
std::cin >> Vector_Student[i].grade;
}
//Sort
std::sort(Vector_Student.begin(), Vector_Student.end());
//Print the required output
for (int j = 0; j < Class_Size; ++j)
{
std::cout
<< Vector_Student[j].name
<< " got a grade of "
<< Vector_Student[j].grade << '\n';
}
return 0;
}
I have an issue with a Vector Struct tutorial.
I'm using Visual Studio 2019 and there's a peculiar scenario where the compiler doesn't give me any warning at all. If I debug it, the first warning appears on line 1544, way out of bounds. The above code will actually sort of compile, run and crash.
I know the issue lies in the sorting but I can't figure it out.

Answer 1

std::sort requires you to implement operator< for your datatype. Here in this case adding following definition in your class will get your code to compile
bool operator<(const StudentDataTypes& that) {
return this->grade < that.grade;
}
Update:
Alternatively as sugested by Casey, we can use custom sort comparator. Here is the sample code for the same.
std::sort(Vector_Student.begin(), Vector_Student.end(), [](const StudentDataTypes& a, const StudentDataTypes&b) { return a.grade < b.grade; });

Answer 2

Here is the answer. I'm posting it in the answers for how to sort a vector-struct.
Step 1:
bool compareTwoStudents(StudentDataTypes a, StudentDataTypes b) {
if (a.grade != b.grade)
return a.grade > b.grade;
return a.grade==b.grade;
}
Step 2:
std::sort(Vector_Student.begin(), Vector_Student.end(),compareTwoStudents);