[c++] error: passing xxx as 'this' argument of xxx discards qualifiers

Let's me give a more detail example. As to the below struct:

struct Count{
    uint32_t c;

    Count(uint32_t i=0):c(i){}

    uint32_t getCount(){
        return c;
    }

    uint32_t add(const Count& count){
        uint32_t total = c + count.getCount();
        return total;
    }
};

As you see the above, the IDE(CLion), will give tips Non-const function 'getCount' is called on the const object. In the method add count is declared as const object, but the method getCount is not const method, so count.getCount() may change the members in count.

Compile error as below(core message in my compiler):

error: passing 'const xy_stl::Count' as 'this' argument discards qualifiers [-fpermissive]

To solve the above problem, you can:

1. change the method uint32_t getCount(){...} to uint32_t getCount() const {...}. So count.getCount() won't change the members in count.

or

2. change uint32_t add(const Count& count){...} to uint32_t add(Count& count){...}. So count don't care about changing members in it.

As to you problem, objects in the std::set are stored as const StudentT, but the method getId and getName are not const, so you give the above error.

You can also see this question Meaning of 'const' last in a function declaration of a class? for more detail.