Learn how to fix error: invalid use of non-static member function.

Question

I am working with cpp and getting the error message:

c.cpp: In member function ‘int Foo::Count(const string&, const string&) const’:
 c.cpp:42:94: error: invalid use of non-static member function
 std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);

Here is the detail of the code that I ran:

class Bar
  {
  public:
  pair<string,string> one;
  std::vector<string> cars;
  Bar(string one, string two, string car);
  };
 
 class Car
  {
  public:
  string rz;
  Bar* owner;
  Car(string car, Bar* p);
  };
 
 class Foo
  {
  public:
  Foo ( void );
  ~Foo ( void );
  int Count ( const string & one, const string & two) const;
  int comparator (const Bar & first, const Bar & second) const; 
  std::vector<Bar> bars;
  };
 
 int Foo::comparator(const Bar & first, const Bar & second) const{
  return first.name < second.name;
 }
 
 int Foo::Count ( const string & one, const string & two ) const{
  int result=0;
  Bar mybar = Bar( one, two, "" );
  std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);
  if (ToFind != bars.end() && ToFind->one == mybar.one ){
  result = ...
  }
  return result;
 }

I need an explanation for the problems I've encountered. How to fix invalid use of non-static member function?

lyytutoria · Accepted Answer

The cause:

int comparator (const Bar & first, const Bar & second) const;

Above function is not a non-static member function because none of the member variables of Foo is used in it, so the error: ''invalid use of non-static member function'' appears.

Solution:

Your error will be solved if Foo::comparator is made static or wrapped in a std::mem_fun object class. lower_bounds() wants the comparer to be a class object which contains a call operator, such as a functor object or function pointer. You can also use a lambda function if C++11 is being used now or in the future. C++11 also possesses std::bind.

For instance:

// Binding:
std::lower_bounds(first, last, value, std::bind(&Foo::comparitor, this, _1, _2));
// Lambda:
std::lower_bounds(first, last, value, [](const Bar & first, const Bar & second) { return ...; });

Zoé Billard · Answer

It is easiest to make the comparator function static.

static int comparator (const Bar & first, const Bar & second);
^^^^^^

Its name will be Foo::comparator when it is invoked in Count.

It is not a non-static function, so it doesn't make sense.

It is possible to make this a non-member operation, especially if you think that Foo might use the comparator.

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Learn how to fix error: invalid use of non-static member function.