Unordered_map insert class C++

Fixed for you:
Please note: C++, unilike C# has NO syntactic sugar for object properties, you have to use standard syntax for method invocation.

#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

class Student
{
private:
  std::string name;
  int mrt_nr;

public:
  void set_name(string);
  void set_nr(int);
  string get_name();
  int get_nr();
};

void Student::set_name(string a)
{
  name = a;
}
void Student::set_nr(int a)
{
  mrt_nr = a;
}

string Student::get_name() {
  return name;
}

int Student::get_nr() {
  return mrt_nr;
}

class Course
{

private:
  string titel_lehrv;
  unordered_map<int, std::string> st;
public:
  Course(const std::string &titel) : titel_lehrv{ titel } {};

  void add( Student &a ) {
    st.insert(pair<int, string>(a.get_nr(), a.get_name()));
  }
};

int main()
{
  pair<int, std::string>s1(12545, "tom");
  Student st;
  st.set_name("tom");
  st.set_nr(123);
  Course mycourse("C++ programming");
  mycourse.add(st);
}

Your code could be more terse:

#include <iostream>
#include <unordered_map>

using namespace std;

class Student
{
private:
  string  name;
  int no;
public:
  Student(string name, int no):name(name), no(no){}
  void set_name(const string & new_name){name = new_name;}
  string get_name() const { return name;}
  void set_no(int  new_no){no = new_no;}
  int get_no() const { return no;}
};

class Course
{
private:
  string title;
  unordered_map < int, string > student_map;
public:
  Course( const string title ) : title(title){}
  void add( const Student & student)
  {
    student_map[student.get_no()] = student.get_name();
  }
  void print() const
  {
    cout << title << "\n";
    for (const auto & [ no, name ] : student_map)
      cout << no << ", " << name << "\n";
  }
};

int main()
{
  Course course("C++ programming");
  course.add( Student("john", 10));
  course.add( Student("jim", 100001));
  course.add( Student("tom", 20210));
  course.print();
}