error: passing xxx as ‘this’ argument of xxx discards qualifiers

by

The objects in the std::set are stored as const StudentT. So when you try to call getId() with the const object the compiler detects a problem, mainly you’re calling a non-const member function on const object which is not allowed because non-const member functions make NO PROMISE not to modify the object; so the compiler is going to make a safe assumption that getId() might attempt to modify the object but at the same time, it also notices that the object is const; so any attempt to modify the const object should be an error. Hence compiler generates an error message.

The solution is simple: make the functions const as:

int getId() const {
    return id;
}
string getName() const {
    return name;
}

This is necessary because now you can call getId() and getName() on const objects as:

void f(const StudentT & s)
{
     cout << s.getId();   //now okay, but error with your versions
     cout << s.getName(); //now okay, but error with your versions
}

As a sidenote, you should implement operator< as : 

inline bool operator< (const StudentT & s1, const StudentT & s2)
{
    return  s1.getId() < s2.getId();
}

Note parameters are now const reference.

More Related Contents:

  1. Differences with const keyword in C++
  2. ‘clrscr()’ and ‘conio.h’ not working
  3. iterating over cases in c++ switch statement? [closed]
  4. How do I read an entire file into a std::string in C++?
  5. Rvalue Reference is Treated as an Lvalue?
  6. Why doesn’t a simple “Hello World”-style program compile with Turbo C++?
  7. How do I use the conditional (ternary) operator?
  8. C++ convert string to hexadecimal and vice versa
  9. Declaring the array size with a non-constant variable
  10. How to parse ini file with Boost
  11. How to create two classes in C++ which use each other as data?
  12. dereferencing the null pointer
  13. What exactly are C++ modules?
  14. How to generate the audio spectrum using fft in C++? [closed]
  15. Why don’t STL containers have virtual destructors?
  16. C++: When (and how) are C++ Global Static Constructors Called?
  17. Destructor being called twice when being explicitly invoked
  18. Is volatile bool for thread control considered wrong?
  19. CMake: failed to run MSBuild command: MSBuild.exe
  20. C++ Equivalent to Designated Initializers?
  21. Any Solution to Unpack a Vector to Function Arguments in C++?
  22. Why do you use std::move when you have && in C++11? [duplicate]
  23. Get an istream from a char*
  24. Visual Studio Code formatting for “{ }”
  25. Should a move constructor take a const or non-const rvalue reference?
  26. Is there a platform or situation where dereferencing (but not using) a null pointer to make a null reference will behave badly?
  27. How to add static libraries to a Visual studio project
  28. Visitor Pattern Explanation
  29. Set local environment variables in C++
  30. Why is pow(int, int) so slow?

Leave a Comment