template comparator not recognized by stl function

[torpid light]

Hi, I tried making a customized comparator for a struct and then do a merge sort with it. but it keeps giving this error:

/usr/include/c++/11/bits/predefined_ops.h:98:22: error: no match for ‘operator<’ (operand types are ‘const myStruct<int>’ and ‘myStruct<int>’)
   98 |       { return __val < *__it; }

My code is like this to merge two parts of same vector.:

template<typename T>
struct myStruct
{
    T key;
    int cnt;
    myStruct() {
        cnt = std::numeric_limits<int>::max();
    }
    bool operator <(const myStruct<T>& rec2) {
        if (key<rec2.key)
            return true;
        else
            return false;
    }
    bool operator >(const myStruct<T>& rec2) {
        if (key>rec2.key)
            return true;
        else
            return false;
    }
};
template <typename T>
struct myStruct_Comparator {
    bool operator()(const myStruct<T>& a, const myStruct<T>& b) const {
        return a.key <= b.key;
    }
};

int main()
{
    typedef int DATATYPE;
    ... ...
    std::vector<myStruct<DATATYPE>> aaa = std::vector<myStruct<DATATYPE>>(100);
    for(int i=0;i<50;i++)
    {
        aaa[i].key = dist(gen); // get random integer
        std::sort(aaa.begin(),(aaa.begin()+50));
    }
    for(int i=50;i<100;i++)
    {
        aaa[i].key = dist(gen); // get random integer
        std::sort(aaa.begin()+50,aaa.end());
    }
    std::cout<<"Unsorted: "<<std::endl;
    for(auto i: aaa) {std::cout<<"Element: "<<i.key<<std::endl;}
    //std::inplace_merge(aaa.begin(), aaa.begin()+50, aaa.end());
    std::inplace_merge(&aaa[0], &aaa[50], &aaa[100]);

    std::cout<<"\nSorted: "<<std::endl;
    for(auto i: aaa) {std::cout<<"Element: "<<i.key<<std::endl;}
}```

Any advices? Thanks  much.

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[torpid light]

template comparator not recognized by stl function

[rose dragon]

you should make your member operator< implementations const

e. g. bool operator <(const myStruct<T>& rec2) const {

[torpid light]

do I also need to add "const" to the operator function "myStruct_Comparator " ?

[rose dragon]

it apears you already did

bool operator()(const myStruct<T>& a, const myStruct<T>& b) const {

in genreal you should always mark member functions that do not modify *this as const to indicate that they can be used on const instances

if you're using this as the comparator for a map or set they have to be so marked because the keys of a map or set are const

[torpid light]

Ah... so it can be think of a good habit to always add const to most of the function {} ?

[rose dragon]

it's generally a good idea to add const to member functions that do not modify *this so that they can be used with const instances

T matches an argument of const T but const T never matches an argument of T

that postpended const is basically changing this from T* to const T* in that member function

[torpid light]

This sounds bit deep to me. So, does it mean if the argument has const elements like in map/set, the compiler will not think it right to pass argument constant to some "T var" in function body, unless the function is marked with const??

[rose dragon]

the compiler won't allow you to pass a pointer or reference to const T to a function that accepts a pointer or reference to non const T because that function is not promising not to try to modify that object

[rapid parrot]

mark the member function const