Passing a lambda function to a generic function C++

Question

I am working on a code using divide and conquer. It just sums the elements of an array: EDIT: Code without inline lamda. Whay I want to is to use a binary operation op so that I can use any binary operation inside the code (not only sum). For example, passing an argument called op:

long reduce_rec(const long *A,long low,long high,myfunclambdahere op)
{
    long result;
    long n = high - low;
    if (n == 0){
      return 0;
    }

    else if(high - low == 1){
      return A[low];
    }
    else{
       long mid=(low+high)/2;
       long a,b;

       a = reduce_rec(A,low,mid);
       b = reduce_rec(A,mid,high);
       //result = a + b;
         result = myfunclambdahere(a,b);
    }
    return result;
}

Now, I want to use this function so that any operator op (binary operator) that is passed to my function can be used. I read that this can be done with a lambda function, but I am not so familiar with it in C++.

they give me the following template

template <class Assoc_comb_op>
value_type reduce_rec(Assoc_comb_op op, long *source, const value_type lo, const value_type hi) {:

But I dont know how to use lambdas and how to code this behavior. Can someone explain it to me?

Thanks in advance

then, how I can achieve this behavior? can you provide code for this? — Edwardo
– Edwardo, Commented Nov 24, 2014 at 1:57
Try adding template <typename myfunclambdahere> in front of your function definition. — didierc
– didierc, Commented Nov 24, 2014 at 4:29
Then somewhere else, define a lambda and pass it to your reduce function: auto mylambda = [x, y] (int x, int y) { return x + y;}; reduce_rec(data,low,high,mylambda);. — didierc
– didierc, Commented Nov 24, 2014 at 4:36
Don't forget to update your function implementation to pass the lambda to recursive calls as well. — didierc
– didierc, Commented Nov 24, 2014 at 4:37

Alexander V · Accepted Answer · 2014-11-24 05:32:40Z

1

Lambda passed as parameter via std::function specification for type safety.

#include <functional>

long reduce_rec(const long *A, long low, long high,
                const std::function<long(long, long)>& op)
{
    long result;
    long n = high - low;
    if (n == 0){
      return 0;
    }

    else if(high - low == 1){
      return A[low];
    }
    else{
       long mid=(low+high)/2;
       long a,b;

       a = reduce_rec(A, low, mid, op);
       b = reduce_rec(A, mid, high, op);
       //result = a + b;
       result = op(a,b);    // lambda call here
    }
    return result;
}


int caller()
{
    auto lambda = [](long a, long b){ return (a + b) * (a - b); };

    return reduce_rec(nullptr /*your data*/, 10, 1110, lambda);
}

answered Nov 24, 2014 at 5:32

Alexander V

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1 Comment

Edwardo Over a year ago

thank you, so if I want to do max, then I use return max(a,b) inside the aut lambda funciton right?

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