C++ sorting a vector of objects, getting error C3867 function call missing argument list?

Question

I'm currently writing a program to detect license plate text. I have a class PossiblePlate with member variables, at one point in main I would like to sort a vector of PossiblePlate objects in descending order by the number of characters detected in the plate, which is in the member variable strChars. Here is the relevant code:

in main:

std::sort(vectorOfPossiblePlates.begin(), vectorOfPossiblePlates.end(), PossiblePlate::sortDescendingByNumberOfChars);

PossiblePlate.h (so far, I'm probably going to add more):

// PossiblePlate.h

#ifndef POSSIBLEPLATE_H
#define POSSIBLEPLATE_H

#include <string>

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>

///////////////////////////////////////////////////////////////////////////////////////////////////
class PossiblePlate {
public:
    // member variables ///////////////////////////////////////////////////////////////////////////
    cv::Mat imgPlate;
    cv::Mat imgGrayscale;
    cv::Mat imgThresh;

    std::vector<cv::RotatedRect> locationOfPlateInScene;

    std::string strChars;

    ///////////////////////////////////////////////////////////////////////////////////////////////////
    bool sortDescendingByNumberOfChars(const PossiblePlate &ppLeft, const PossiblePlate &ppRight);

};

#endif      // end #ifndef POSSIBLEPLATE_H

here is PossiblePlate.cpp (so far, I may add more)

// PossiblePlate.cpp

#include "PossiblePlate.h"


///////////////////////////////////////////////////////////////////////////////////////////////////
bool sortDescendingByNumberOfChars(const PossiblePlate &ppLeft, const PossiblePlate &ppRight) {
    return(ppLeft.strChars.length() < ppRight.strChars.length());
}

When I run this with Visual Studio 2013 I get the following error:

Error   1   error C3867: 'PossiblePlate::sortDescendingByNumberOfChars': function call missing argument list; use '&PossiblePlate::sortDescendingByNumberOfChars' to create a pointer to member c:\visualstudio2013progs\cpp\licenseplaterecognition1\main.cpp  44  1   LicensePlateRecognition

Line 44 in main is the line in main above that calls std::sort.

I've done the same thing in other programs before without a problem. Everything I'm doing here is consistent with how I have done this in the past and with other Stack Overflow posts such as these:

sorting a vector of structs

sort vector of struct element

Sorting a vector of objects in C++

Can anybody tell me what I'm doing wrong? I'm at a loss here, any help would be greatly appreciated.

Pete Becker · Accepted Answer · 2015-11-27 02:43:48Z

4

There are several problems in the posted code.

First, the function passed to std::sort has to be callable with two arguments of the type being sorted. PossiblePlate::sortDescendingByNumberOfChars is a non-static member function, so it has to be called on an object of type PossiblePlate, with two additional arguments. std::sort can't cope with that. Make it a static member function.

Second, PossiblePlate.cpp defines a free function named sortDescendingByNumberOfChars; that's okay, but it's not the member function. Change its declaration from sortDescendingByNumberOfChars to PossiblePlate::sortDescendingByNumberOfChars to define the member function.

Third, follow the advice in the error message. To take the address of a member function, the correct syntax is &ClassName::FunctionName. The call in the code does not have the &.

answered Nov 27, 2015 at 2:43

Pete Becker

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3 Comments

rcgldr Over a year ago

As answered, std::sort won't work with non-static member functions. Alternatives to using a static function are a lambda function or a functor (function operator).

Pete Becker Over a year ago

@rcgldr - or with a free function, like the one defined in PossiblePlate.cpp. But that's not what this question is about.

rcgldr Over a year ago

I just wanted to warn the OP about the potential issue and alternatives.

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