Error with lambda in template member function - c++11

Question

I have the following c++ code
#include <iostream>
template<typename Func>
class Foo
{
private:
Func func;
public:
Foo(Func func) : func(func) {}
template<typename T>
Func wrap()
{
Func clbk = func;
auto wrapperCB = [clbk](T t) {
auto job = [clbk, t](){
clbk(t);
};
job();
};
return wrapperCB;
}
template<typename T>
void call(T t)
{
func(t);
}
};
int main()
{
int m = 2;
auto f = [](int & p) {std::cout << "test success " << p << "\n";};
auto obj = std::make_shared<Foo<std::function<void(int &)>>>(f);
auto wrapper = obj->template wrap<int &>();
wrapper(m);
return 0;
}
This is giving compilation error
tsavs-mbp:p utsagarw$ clear; g++ -std=c++11 a.cpp -o z; ./z
a.cpp:18:17: error: no matching function for call to object of type 'const std::__1::function<void (int &)>'
clbk(t);
^~~~
a.cpp:38:32: note: in instantiation of function template specialization 'Foo<std::__1::function<void (int &)> >::wrap<int &>' requested here
auto wrapper = obj->template wrap<int &>();
^
/Applications/Xcode_10.1/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/functional:1677:9: note: candidate function not viable: 1st argument ('const int') would lose const qualifier
_Rp operator()(_ArgTypes...) const;
^
1 error generated.
I don't understand this error. Where did this const come from?
It is building successfully if in wrap I don't create job functor and call clbk directly. What is this job doing to type T?
template<typename T>
Func wrap()
{
Func clbk = func;
auto wrapperCB = [clbk](T t) {
clbk(t);
};
return wrapperCB;
}

Answer 1

If you want to modify any captured variable inside lambda you have to specify it as mutable.
t variable is captured by copy, so you can only read it:
auto job = [clbk, t]() // <-- t passed by copy
{
clbk(t); // clbk takes t by reference -> int&
};
your callback, clbk has signature int& so it means it could modify t. What is not allowed.
Solution:
auto job = [clbk, t]() mutable // keyword 'mutable' added
{
clbk(t); // clbk can change t
};
or make function taking const int& as parameter - then t can be only read.
Demo