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

#include <iostream>
#include <set>

using namespace std;

class StudentT {

public:
    int id;
    string name;
public:
    StudentT(int _id, string _name) : id(_id), name(_name) {
    }
    int getId() {
        return id;
    }
    string getName() {
        return name;
    }
};

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

int main() {

    set<StudentT> st;
    StudentT s1(0, "Tom");
    StudentT s2(1, "Tim");
    st.insert(s1);
    st.insert(s2);
    set<StudentT> :: iterator itr;
    for (itr = st.begin(); itr != st.end(); itr++) {
        cout << itr->getId() << " " << itr->getName() << endl;
    }
    return 0;
}

In line:

cout << itr->getId() << " " << itr->getName() << endl;

It give an error that:

../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

What's wrong with this code? Thank you!

Best Answer

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.

Related Solutions

C++ – An odd C++ error: test.cpp:15: error: passing ‘const ’ as ‘this’ argument of ‘’ discards qualifiers

The problem is calling a non-const function test2.test() on a const object test2 from testing::test1.

testing::test1 gets test2 as a parameter const testing &test2. So within testing::test1, test2const. Then in the first line of the function:

test2.test()

The testing::test function is called on test2. That function is not declared with const at the signature end, so it may modify the object it is called on (the this pointer implicitly passed to it), and even though it does not, the compiler assumes so. By letting you call it there, the compiler would let you modify a const variable without an explicit cast, which C++ is not supposed to allow. Therefore to explain the error message:

test.cpp:15: error: passing ‘const testing’ as ‘this’ argument of ‘int testing::test()’ discards qualifiers

this refers to the object the member function (testing::test) operates on, and in this case it is not const, because testing::test was not declared with const, and thus the mismatch is detected when trying to make a non-const pointer (this) refer to a const object (testing), ignoring the const qualifier.

To solve this, decide whether the testing::test function should ever need to modify the object it is called on (the way it is written now it does not, as all it does is return 1, however that may change, so you need to think at what its intended functionality is). If it should, then obviously calling it on a const object is bad, although you can use const_cast to ask the compiler to override that, but this is dangerous. If it should not, then mark it const, so that it can be called on const objects as well:

class testing{
    int test1() const;
    // ...
}

int testing::test() const {
    // ...
}

Best Answer

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C++ – An odd C++ error: test.cpp:15: error: passing ‘const *’ as ‘this’ argument of ‘*’ discards qualifiers

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C++ – An odd C++ error: test.cpp:15: error: passing ‘const ’ as ‘this’ argument of ‘’ discards qualifiers