How Can I Use a Member Function as a Comparator in C `std::sort`?

Implementing Member Function Comparators for Sorting

In C , sorting a collection requires a comparator function. Attempting to utilize a member function as a comparator may result in a compilation error.

Problem

Consider the following class:

class MyClass {
    int * arr;
    // ...

    doCompare( const int & i1, const int & i2 ) { // uses member variables } 

    doSort() { std::sort(arr,arr+someSize, &doCompare); }
};

Compiling this code may generate an error:

ISO C forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function.

Solution

To resolve this issue, one approach is to make doCompare a static member function. However, if doCompare requires access to MyClass data, you can convert MyClass into a comparison functor.

This can be achieved by changing:

doCompare( const int & i1, const int & i2 ) { // use some member variables }

to:

bool operator () ( const int & i1, const int & i2 ) { // use member variables }

Then, call the sort function as follows:

doSort() { std::sort(arr, arr+someSize, *this); }

Additionally, the doSort method should return a value, such as void.

Another option is to wrap the member function within the class, as demonstrated in the following example:

class MyClass {
    struct Less {
        Less(const MyClass& c) : myClass(c) {}
        bool operator () ( const int & i1, const int & i2 ) {// use 'myClass'} 
        MyClass& myClass;
    };
    doSort() { std::sort(arr, arr+someSize, Less(*this)); }
};

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