CODEWITHSUNDEEP
c++stliterator
Q-bertsuit
Q-bertsuit

Reputation: 3437

Compile error using adjacent_difference

I'm trying to use adjacent_difference with two different iterator types. The functor I have created takes the type used by the InputIterator as parameters and returns the type used by the OutputIterator. I don't understand why the code I included doesn't compile:

In file included from /usr/include/c++/4.9/numeric:62:0, from 4: /usr/include/c++/4.9/bits/stl_numeric.h: In instantiation of '_OutputIterator std::adjacent_difference(_InputIterator, _InputIterator, _OutputIterator, _BinaryOperation) [with _InputIterator = __gnu_cxx::__normal_iterator >; _OutputIterator = __gnu_cxx::__normal_iterator >; _BinaryOperation = {anonymous}::CheckOp]': 48:85: required from here /usr/include/c++/4.9/bits/stl_numeric.h:374:17: error: cannot convert '_ValueType {aka Test}' to 'float' in assignment *__result = __value;

// Example program
#include <iostream>
#include <string>
#include <numeric>
#include <vector>

struct Test
{
    float first;
    float second;

};

namespace{
    class CheckOp {
    public:

        float operator()(Test x, Test y) const
        {
            float a = x.first - y.first;
            float b = x.second - y.second;

            return a + b;

        }
    };
}

int main()
{
    std::vector<Test> testVec;

    Test test1;
    test1.first = 5.5F;
    test1.second = 6.5F;
    testVec.push_back(test1);
    Test test2;
    test2.first = 2.5F;
    test2.second = 8.5F;
    testVec.push_back(test2);
    Test test3;
    test3.first = 9.4F;
    test3.second = 7.8F;
    testVec.push_back(test3);

    CheckOp checkOP;
    std::vector<float> resultVec(testVec.size());

    std::adjacent_difference(testVec.begin(), testVec.end(), resultVec.begin(), checkOP);


}

Upvotes: 1

Views: 435

Answers (1)

Sebastian Redl
Sebastian Redl

Reputation: 72054

Note the description of adjacent_difference (from cppreference):

First, creates an accumulator acc whose type is InputIt's value type, initializes it with *first, and assigns the result to *d_first.

This implies that the input and output sequences must have the same, or at least a compatible, type.

Upvotes: 3

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