Why Do Non-Const Member Functions Cause Errors When Used with Const Objects in a `std::set`?

Error in Passing Non-Const Member Functions to Const Objects

In the provided code, a std::set is used to store StudentT objects. The issue arises when attempting to call the getId() and getName() member functions on objects within the set. These member functions are not marked as const, which means they may modify the object's data. However, the objects in the std::set are stored as const StudentT, preventing any modifications.

The compiler detects this inconsistency and generates the following error messages:

../main.cpp:35: error: passing 'const StudentT' as 'this' argument of 'int StudentT::getId()' discards qualifiers
../main.cpp:35: error: passing 'const StudentT' as 'this' argument of 'std::string StudentT::getName()' discards qualifiers

This error indicates that the compiler is attempting to pass a const object as the this argument to non-const member functions, which is prohibited.

Solution:

To resolve this issue, modify the getId() and getName() member functions to be const as follows:

int getId() const {
    return id;
}

string getName() const {
    return name;
}

By marking these functions as const, you ensure that they will not modify the object's data and can be safely called on const objects.

Additionally, it is also recommended to make the < operator comparison function const by passing const references to its parameters:

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

These modifications will prevent unexpected errors and ensure that the code operates correctly.

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