error: passing xxx as 'this' argument of xxx discards qualifiers
C++C++ Problem Overview
#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!
C++ Solutions
Solution 1 - C++
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.
Solution 2 - C++
Member functions that do not modify the class instance should be declared as const
:
int getId() const {
return id;
}
string getName() const {
return name;
}
Anytime you see "discards qualifiers", it's talking about const
or volatile
.
Solution 3 - C++
Actually the C++ standard (i.e. C++ 0x draft) says (tnx to @Xeo & @Ben Voigt for pointing that out to me):
> 23.2.4 Associative containers
> 5 For set and multiset the value type
> is the same as the key type. For map
> and multimap it is equal to pair
> 6 iterator of
> an associative container is of the
> bidirectional iterator category. For
> associative containers where the value
> type is the same as the key type, both
> iterator and const_iterator are
> constant iterators. It is unspecified
> whether or not iterator and
> const_iterator are the same type.
So VC++ 2008 Dinkumware implementation is faulty.
Old answer:
You got that error because in certain implementations of the std lib the set::iterator
is the same as set::const_iterator
.
For example libstdc++ (shipped with g++) has it (see here for the entire source code):
typedef typename _Rep_type::const_iterator iterator;
typedef typename _Rep_type::const_iterator const_iterator;
And in SGI's docs it states:
iterator Container Iterator used to iterate through a set.
const_iterator Container Const iterator used to iterate through a set. (Iterator and const_iterator are the same type.)
On the other hand VC++ 2008 Express compiles your code without complaining that you're calling non const methods on set::iterator
s.
Solution 4 - C++
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:
- change the method
uint32_t getCount(){...}
touint32_t getCount() const {...}
. Socount.getCount()
won't change the members incount
.
or
- change
uint32_t add(const Count& count){...}
touint32_t add(Count& count){...}
. Socount
don't care about changing members in it.
As to your 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 https://stackoverflow.com/questions/751681/meaning-of-const-last-in-a-function-declaration-of-a-class for more detail.