how to use function object with an array?

Question

I want to write a C++ generic algorithm that can work on set, list, map, vector, etc... So this algorithm accepts a container and a function object, so I can check for certain conditions.

This algorithm needs to check the longest series in the container according to a certain condition and return its length.

i am having trouble in passing the function object to the function as i get this error (in the second line in main) :

"Multiple markers at this line - Invalid arguments ' Candidates are: void functionA(#0 &, #0 &, #1) ' - invalid initialization of non-const reference of type 'int*&' from an rvalue of type 'std::array::iterator {aka "

i don't understand the problem since there is an operator () , in the function object :\ even if i call the constructor first i still get this error ..

what i tried :

template<typename T,typename Predicate>
void functionA(T& it1 , T& end1,Predicate pred){
    for(;it1 != end1; ++it1){
        T it2=it1++;
        if(!pred(*it1,*it2)){
            std::cout << *it1 << "\n" ;
            return;
        }
        }
    std::cout <<"not found" << "\n" ;
}

class FindFirst {
    public:
    FindFirst();
    bool operator()(int f , int s) const {
       return f < s;
    }
};

int main() {

    std::array<int,11> myarray = {1,2,4,7,10,14,3,6,12,24,48};
    functionA(myarray.begin(),myarray.end(),find);

}

songyuanyao · Accepted Answer · 2018-01-25 03:51:29Z

2

The problem is that std::array::begin returns by value, which is a temporary and can't be bound to lvalue reference to non-const (i.e. T&).

Change the parameter type of functionA to pass-by-value, e.g.

template<typename T,typename Predicate>
void functionA(T it1 , T end1,Predicate pred){

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edited Jan 25, 2018 at 3:51

answered Jan 25, 2018 at 3:42

songyuanyao

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3 Comments

mamama Over a year ago

i get this error now : undefined reference to `FindFirst::FindFirst()'

songyuanyao Over a year ago

@mamama You need to define it, the constructor. If you have nothing to implement you can just remove the declaration.

songyuanyao Over a year ago

@mamama Congratulations; at least you got advances.

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