invalid use of non-static member function

Question

I have something like this    class Bar               public          pair lt string string gt  one          std  vector lt string gt  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  amp  one  const string  amp   two  const            int comparator  const Bar  amp  first  const Bar  amp  second  const                          std  vector lt Bar gt  bars             int Foo  comparator const Bar  amp  first  const Bar  amp  second  const    return first name  lt  second name     int Foo  Count    const string  amp  one  const string  amp  two   const    int result 0    Bar mybar   Bar  one  two          std  vector lt Bar gt   iterator ToFind   lower bound bars begin    bars end    mybar  comparator     if  ToFind    bars end    amp  amp  ToFind- gt one    mybar one        result             return result      The method Foo  Count should use std  lower bound   to find element in vector lt Bar gt  according to pair of two strings   Now the part which doesn t work  To lower bound   I m providing method comparator    I thought it was okay  but g   says    c cpp  In member function    int Foo  Count const string amp   const string amp   const     c cpp 42 94  error  invalid use of non-static member function std  vector lt Bar gt   iterator ToFind   lower bound bars begin    bars end    mybar  comparator     And the method Count   must stay const     I m quite new to C   because I m forced to learn it    Any ideas

User · Answer

The simplest fix is to make the comparator function be static:

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

When invoking it in Count, its name will be Foo::comparator.

The way you have it now, it does not make sense to be a non-static member function because it does not use any member variables of Foo.

Another option is to make it a non-member function, especially if it makes sense that this comparator might be used by other code besides just Foo.

User · Answer

You shall pass a this pointer to tell the function which object to work on because it relies on that as opposed to a static member function

User · Answer

You must make Foo  comparator static or wrap it in a std  mem fun class object   This is because lower bounds   expects the comparer to be a class of object that has a call operator  like a function pointer or a functor object   Also  if you are using C  11 or later  you can also do as dwcanillas suggests and use a lambda function   C  11 also has std  bind too   Examples      Binding  std  lower bounds first  last  value  std  bind  amp Foo  comparitor  this   1   2       Lambda  std  lower bounds first  last  value     const Bar  amp  first  const Bar  amp  second    return

[c++] invalid use of non-static member function

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