Passing C++ lambda as argument to non templated function

Question

As I googled std::function works slower than simple lambda function.

Consider the following use case will there be std::function penalty when using stl sorting algorithms (v vector may be big enough and occupy about N*GB of data):

#include <iostream>
#include <functional>
#include <algorithm>
#include <vector>

using namespace std;

void sorter(std::vector<int>& v, std::function<bool(int a, int b)> f){
    std::sort(v.begin(), v.end(), f);
}

int main()
{
    std::vector<int> v = {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
    for (auto val : v)
        cout << val << " ";
    cout << endl;

    int c = 4;
    auto f = [&c](int a, int b){ return a+b < c; };
    
    sorter(v, f);

    for (auto val : v)
        cout << val << " ";
    cout << endl;

    return 0;
}

As you can see I create the lambda function f and pass it to a sorter function. My task is to keep sorter function non-templated that is why I tried to pass lamda function as std::function. Or are there better ways of doing that?

Since the lambda function is non-capturing, you could have the parameter accept a function pointer instead. — Eljay
– Eljay, Commented Aug 6, 2022 at 14:22
Non-captured lambdas can be converted to function pointers, so you can directly void sorter(std::vector<int>& v, bool(*f)(int, int)). — 康桓瑋
– 康桓瑋, Commented Aug 6, 2022 at 14:30
No, with a capture your options are to use a template or a std::function (or another type erasure similar to std::function). — aschepler
– aschepler, Commented Aug 6, 2022 at 14:59
You can create your own type erasure that is lighter weight than std::function. Example. — Raymond Chen
– Raymond Chen, Commented Aug 6, 2022 at 15:16
As far as I understand the performance issue, a template might be faster because the compiler is probably inline the code into the algorithm. In practice unless you have to sort large amount of data with trivial comparison, the overhead should not matter much. — Phil1970
– Phil1970, Commented Aug 6, 2022 at 15:41

Aykhan Hagverdili · Accepted Answer · 2022-08-06 14:57:14Z

3

Lambdas that do not capture anything can be converted to a function pointer with the + operator:

void sorter(std::vector<int>& v, bool (*cmp)(int,int));

int main() {
  auto const cmp = +[](int a, int b) { return a < b; };
  vector<int> v{3, 2, 1};
  sorter(v, cmp);
}

But if it does capture something, you should either make it a template, or use std::function. No way around that.

answered Aug 6, 2022 at 14:57

Aykhan Hagverdili

30.5k6 gold badges53 silver badges108 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Passing C++ lambda as argument to non templated function

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related