Code in C++ that won't even compile, but everything seems right

Question

This simple program just won't even compile because of a line in which I intend to add something to a vector. Any idea on why?

#include <iostream>
#include <vector>

class Subject
{
private:
    std::string name;
    std::vector<int> sectionPercentages;

public:
    Subject()
    {
        std::cout << "Write the name of the subject: ";
        std::cin >> this -> name;
        int percentage;
        
        while(true){
            std::cout << "Write a new section name and its percentage with no symbols: ";
            std::cin >> percentage;
            this -> sectionPercentages.push_back(percentage);
        }
    }
};

int main()
{
    std::vector<Subject> subjects;

    while(true)
    {
        Subject newSubject();
        subjects.push_back(newSubject);
    }
    
    return 0;
}

I tried refactoring the code, commenting the body of the constructor, etc... Does not work...

There is a code I did when I was 13 as mentioned, which I used as a reference to create this one:

#include <iostream>
#include <cmath>
#include <vector>
#include <string>

class PhysicsVector
{
private:
    std::string name;
    float magnitude;
    float direction;
    float angle;
    float xCoord;
    float yCoord;

public:
    PhysicsVector(std::string name)
    {
        this -> name = name;
        this -> magnitude = 0;
        this -> direction = 0;
        this -> angle = 0;
        this -> xCoord = 0;
        this -> yCoord = 0;
    }
    
    PhysicsVector(std::string name, float magnitude, float direction)
    {
        this -> name = name;
        this -> magnitude = magnitude;
        this -> direction = direction;
        calculateCoords();
    }
    
    PhysicsVector(std::string name, float xCoord, float yCoord, int any)
    {
        this -> name = name;
        this -> xCoord = xCoord;
        this -> yCoord = yCoord;
    }

    void calculateCoords()
    {
        this -> angle = this -> direction;
        
        if (direction > 90 && direction < 180)
        {
            this -> angle -= 90;
        }else if (direction > 180 && direction < 270)
        {
            this -> angle -= 180;
        }else if (direction > 270 && direction < 360)
        {
            this -> angle -= 270;
        }
        
        this -> angle *= (M_PI / 180);

        if (this -> direction == 0)
        {
            this -> xCoord = this -> magnitude;
            this -> yCoord = 0;
        }else if (direction == 90)
        {
            this -> xCoord = 0;
            this -> yCoord = this -> magnitude;
        }else if (direction == 180)
        {
            this -> xCoord = this -> magnitude * -1;
            this -> yCoord = 0;
        }else if (direction == 270)
        {
            this -> xCoord = 0;
            this -> yCoord = this -> magnitude * -1;
        }else if (direction > 0 && direction < 90)
        {
            this -> xCoord = (this -> magnitude * cos(this -> angle));
            this -> yCoord = (this -> magnitude * sin(this -> angle));
        }else if (direction > 90 && direction < 180)
        {
            this -> xCoord = (this -> magnitude * sin(this -> angle)) * -1;
            this -> yCoord = (this -> magnitude * cos(this -> angle));
        }else if (direction > 180 && direction < 270)
        {
            this -> xCoord = (this -> magnitude * cos(this -> angle)) * -1;
            this -> yCoord = (this -> magnitude * sin(this -> angle)) * -1;
        }else if (direction > 270 && direction < 360)
        {
            this -> xCoord = (this -> magnitude * sin(this -> angle));
            this -> yCoord = (this -> magnitude * cos(this -> angle)) * -1;
        }
    }
    
    void describe()
    {
        std::cout << "Name: " << this -> name << std::endl;
        std::cout << "Magnitude: " << this -> magnitude << std::endl;
        std::cout << "Direction: " << this -> direction << std::endl;
        std::cout << "Position in x: " << this -> xCoord << std::endl;
        std::cout << "Position in y: " << this -> yCoord << std::endl;
    }
    
    void edit(std::string name, float magnitude, float direction)
    {
        this -> name = name;
        this -> magnitude = magnitude;
        this -> direction = direction;
        calculateCoords();
    }
    
    void sumVector(PhysicsVector anotherVector)
    {
        this -> xCoord += anotherVector.xCoord;
        this -> yCoord += anotherVector.yCoord;
        this -> magnitude = sqrt((xCoord * xCoord) + (yCoord * yCoord));
        
        if (this -> xCoord > 0 && yCoord == 0)
        {
            this -> direction = 0;
        }else if (this -> xCoord == 0 && this -> yCoord > 0)
        {
            this -> direction = 90;
        }else if (this -> xCoord < 0 && this -> yCoord == 0)
        {
            this -> direction = 180;
        }else if (this -> xCoord == 0 && this -> yCoord < 0)
        {
            this -> direction = 270;
        }

        if (this -> xCoord > 0 && this -> yCoord > 0)
        {
            this -> direction = atan(yCoord / xCoord) * 180 / M_PI;
        }else if (this -> xCoord < 0 && this -> yCoord > 0)
        {
            this -> direction = atan(xCoord / yCoord) * 180 / M_PI;
        }else if (this -> xCoord < 0 && this -> yCoord < 0)
        {
            this -> direction = atan(yCoord / xCoord) * 180 / M_PI;
        }else if (this -> xCoord > 0 && this -> yCoord < 0)
        {
            this -> direction = atan(xCoord / yCoord) * 180 / M_PI;
        }
    }
};

int main(int argc, char const *argv[])
{
    std::vector<PhysicsVector> createdVectors;
    PhysicsVector newVector("VOID");
    PhysicsVector resultVector("SUM");

    std::string name;
    float magnitude;
    float direction;
    
    int counter = 0;

    std::cout << "Name of the new vector: ";
    std::cin >> name;
    std::cout << "Magnitude of the new vector: ";
    std::cin >> magnitude;
    std::cout << "Direction of the new vector: ";
    std::cin >> direction;
    newVector.edit(name, magnitude, direction);
    createdVectors.push_back(newVector);

    while (true)
    {
        system("cls");
        
        std::cout << std::endl << "Created vectors:" << std::endl;
        
        counter = 0;
        
        while (counter < createdVectors.size())
        {
            std::cout << std::endl;
            createdVectors.at(counter).describe();
            std::cout << std::endl;
            counter += 1;
        }
        
        if (counter > 1)
        {
            std::cout << "Name of the new vector (\"SUM\" to sum all vectors): ";
        }else
        {
            std::cout << "Name of the new vector: ";
        }

        std::cin >> name;

        if (name == "SUM")
        {
            counter = 0;

            while (counter < createdVectors.size())
            {
                resultVector.sumVector(createdVectors.at(counter));
                counter += 1;
            }

            break;
        }

        std::cout << "Magnitude of the new vector: ";
        std::cin >> magnitude;
        std::cout << "Direction of the new vector: ";
        std::cin >> direction;
        newVector.edit(name, magnitude, direction);
        createdVectors.push_back(newVector);
    }

    std::cout << std::endl;
    resultVector.describe();
    
    system("pause");
}