Home/ Questions/Learn how to fix error: invalid use of non-static member function.
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River Wang
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Asked: 2022-05-18T17:09:51+00:00 In:

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

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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?

invalid use
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2 Answers

  1. Best Answer
    Expert

    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 ...; });
    • 0

  2. 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.

    • 23

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