Why is there a conversion in this code even though I'm not doing one?

Question

I have just finished a course on c++ at university, and we got our final assignment. (I just want to clearify: this is in no way me asking you to do my homework for me, I just need some help with the code.)

Our first part of the assignment is to write a generic function that gets a functor and 2 indices: the functor is a condition, and the indices point to the first element in a container and the place after the last element in a container. The function needs to check how many pairs of elements in the container meet the condition of the functor.

This is my code:

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

bool test(vector<int> v);
template<typename InputIterator, typename Condition>
int checkCondition(InputIterator first, InputIterator last, Condition condition);

class bigger{
public:
    bigger() = default;
    ~bigger = default();
    bigger(const bigger&) = default;
    bigger& operator=(const bigger&) = default;
    bool operator()(int a, int b) const {
        return (a<b);
    }
};

template<typename InputIterator, typename Condition>
int checkCondition(InputIterator first, InputIterator last, Condition condition){
    int counter = 0;
    while (first!=last){
        vector<int>::iterator temp = first;
        ++temp;
        if (condition(*first, *temp)){
            counter++;
        }
        ++first;
    }
    return counter;
}

bool test(vector<int> v){
    vector<int>::iterator first = v.begin();
    vector<int>::iterator last = v.end();
    bigger condition();
    return checkCondition(first, last, condition);
}

And here is the error that I get by compiling with g++ using Linux's terminal (ubuntu because I'm using windows):

part1OfDry.cpp: In instantiation of ‘int checkCondition(InputIterator, InputIterator, Condition) [with InputIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; Condition = bigger (*)()]’:
part1OfDry.cpp:41:49:   required from here
part1OfDry.cpp:29:22: error: too many arguments to function
         if (condition(*first, *temp)){
             ~~~~~~~~~^~~~~~~~~~~~~~~
part1OfDry.cpp:29:22: error: could not convert ‘condition()’ from ‘bigger’ to ‘bool’

And this is my compilation line: g++ -o test1.exe -std=c++11 -Wall -pedantic-errors -Werror -DNDEBUG thing.cpp

If someone can please explain why the errors are happening, I'll be very thankful.

it is "functor" from "function" – 463035818_is_not_an_ai Feb 06 '20 at 14:16 — 463035818_is_not_an_ai, Feb 06 '20 at 14:16
look up "most vexing parse" – user253751 Feb 06 '20 at 14:16 — user253751, Feb 06 '20 at 14:16
Remove `()` from `bigger condition();` – ChrisMM Feb 06 '20 at 14:17 — ChrisMM, Feb 06 '20 at 14:17
fanctor? not a c++ term – Adrian Cornish Feb 06 '20 at 14:18 — Adrian Cornish, Feb 06 '20 at 14:18
Thank you very much, @ChrisMM. – Gil Levin Feb 06 '20 at 14:19 — Gil Levin, Feb 06 '20 at 14:19

max66 · Answer 1 · 2020-02-06T14:21:08.867

4

Try writing

bigger condition{};

or, as suggested by 0x5453, simply

bigger condition;

instead of

bigger condition();

Problem: as you can see from the error message, bigger condition(); is interpreted

 Condition = bigger (*)()

as declaration of a function returning a bigger.

Search for "most vexing parse" for clarifications.

edited Feb 06 '20 at 14:21

answered Feb 06 '20 at 14:19

max66

65,235
10
71
111

Or simply `bigger condition;`. – 0x5453 Feb 06 '20 at 14:20
@0x5453 - yes, it's simpler; added; thanks. – max66 Feb 06 '20 at 14:24
His default destructor also has extra brackets – Adrian Cornish Feb 06 '20 at 14:24
@AdrianCornish - I didn't seen it... not extra brackets but brackets in the wrong place. IMHO, you should explain, in your answer, the exact place were the parentheses are wrong. – max66 Feb 06 '20 at 14:31
@max66 Which is why I posted the code - if you cannot diff two files then what can I do? Plus I saw it because I actually tried compiling his code which you obviously did not do – Adrian Cornish Feb 06 '20 at 14:32
@AdrianCornish - I think you could simply write the wrong line (`~bigger = default();`), explaining that is wrong and why, and propose the correct line. – max66 Feb 06 '20 at 14:34
@max66 Or I could post the complete correct code? What is you point Max? – Adrian Cornish Feb 06 '20 at 14:36
1

@AdrianCornish - the point is that from your answer I don't understand were are the not needed parentheses; from your first comment I understand it. IMHO, your first comment is a better answer than your answer. – max66 Feb 06 '20 at 14:38

score -1 · Answer 2 · answered Feb 06 '20 at 14:22

Stop adding parentheses where they are not needed.

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

bool test(vector<int> v);
template<typename InputIterator, typename Condition>
int checkCondition(InputIterator first, InputIterator last, Condition condition);

class bigger{
public:
    bigger() = default;
    ~bigger() = default;
    bigger(const bigger&) = default;
    bigger& operator=(const bigger&) = default;
    bool operator()(int a, int b) const {
        return (a<b);
    }
};

template<typename InputIterator, typename Condition>
int checkCondition(InputIterator first, InputIterator last, Condition condition){
    int counter = 0;
    while (first!=last){
        vector<int>::iterator temp = first;
        ++temp;
        if (condition(*first, *temp)){
            counter++;
        }
        ++first;
    }
    return counter;
}

bool test(vector<int> v){
    vector<int>::iterator first = v.begin();
    vector<int>::iterator last = v.end();
    bigger condition;
    return checkCondition(first, last, condition);
}

Too terse and too verbose at the same time. That's talent. – Mark Storer Feb 06 '20 at 14:54 — Mark Storer, Feb 06 '20 at 14:54

Why is there a conversion in this code even though I'm not doing one?

2 Answers2