How to negate a lambda function result - c++

Question

In the following there is a code snippet written in C++ which does not compile.
The reason is trying to reverse the result of a lambda function using not1(). I'll appreciate very much if someone could fix this code
#include <iostream> // std::cout
using namespace std;
#include <functional> // std::not1
#include <algorithm> // std::count_if
#include <vector>
int main () {
vector<int> sv = {3, 5, 10,12 };
vector<int> v = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 };
auto validSelection = [&](auto& e) {
auto isSelected = [&] (auto& sve) {
return e == sve;
};
return find_if(sv.begin(), sv.end(), isSelected) != sv.end();
};
stable_partition(v.begin(), next(v.begin(),8) , not1(validSelection));
for (int n : v) {
std::cout << n << ' ';
}
std::cout << '\n';
return 0;
}

Answer 1

One of approaches is to use the wrapper std::function. For example
auto even = [](int x) { return x % 2 == 0; };
std::cout << std::not1(std::function<bool(int)>(even))(11) << std::endl;
The function object adapter not1 requires that the corresponding predicate used as argument had typedef names argument_type and result_type and the wrapper std::function provides them.
In your case the equivalent call would look like this:
stable_partition(v.begin(), next(v.begin(), 8) , not1(function<bool(int&)>(validSelection)));
Demo.
Or if I have understood correctly what you are trying to do then the corresponding code can look as it is shown in the following demonstrative program
#include <iostream>
#include <vector>
#include <functional>
#include <algorithm>
int main()
{
std::vector<int> sv = { 3, 5, 10, 12 };
std::vector<int> v = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 };
auto validSelection = [&](int x)
{
return std::binary_search(sv.begin(), sv.end(), x);
};
std::stable_partition(v.begin(), v.end(), validSelection);
for (int x : v) std::cout << x << ' ';
std::cout << std::endl;
std::stable_partition(v.begin(), v.end(), std::not1( std::function<bool( int )>( validSelection) ) );
for (int x : v) std::cout << x << ' ';
std::cout << std::endl;
return 0;
}
In this case the output is
3 5 10 12 1 2 4 6 7 8 9 11 13 14 15
1 2 4 6 7 8 9 11 13 14 15 3 5 10 12
Pay attention to using of the algorithm std::binary_search because the vector sv is sorted.