Lambda of a lambda : the function is not captured

Question

The following program do not compile :

#include <iostream> #include <vector> #include <functional> #include <algorithm> #include <cstdlib> #include <cmath>  void asort(std::vector<double>& v, std::function<bool(double, double)> f) {     std::sort(v.begin(), v.end(), [](double a, double b){return f(std::abs(a), std::abs(b));}); }  int main() {     std::vector<double> v({1.2, -1.3, 4.5, 2.3, -10.2, -3.4});     for (unsigned int i = 0; i < v.size(); ++i) {         std::cout<<v[i]<<" ";     }     std::cout<<std::endl;     asort(v, [](double a, double b){return a < b;});     for (unsigned int i = 0; i < v.size(); ++i) {         std::cout<<v[i]<<" ";     }     std::cout<<std::endl;     return 0; } 

because :

error : 'f' is not captured 

What does it mean and how to solve the problem ?

Answer 1

You use the f parameter in the lambda inside asort(), but you don't capture it. Try adding f to the capture list (change [] to read [&f]).

Answer 2

You are effectively referencing f, which is a variable in the outer scope, in your lambda. You should capture it in your capture list (simplest is probably by reference [&f], or [&] to capture everything by reference, as you are using it immediately).

On another note, std::function has some overhead as it performs type erasure, in your case here it might be better to introduce a template type.