Cannot capture reference in lambda from universal reference? - c++

Question

I am trying to figure out why the captured variable cannot be passed to the foo function by reference, the program shown below does not compile with the following error:
error: invalid initialization of reference of type
'Object&' from expression of type 'const Object'
Is there some way to perfect forward the universal reference into the lambda so that it is captured by reference for later use? Types T might also be R-Values or a combination of R-Values and L-Values, so they cannot always be captured by reference.
struct Object
{
};
void foo(Object& a)
{
}
template<typename... T>
auto bar(T&&... values)
{
return [values...](){
foo(values...);
};
}
int main ()
{
Object obj;
auto lambda = bar(obj);
return 0;
}

Answer 1

You're by capturing by values which stores values as const members inside the closure, and they cannot bind to non-const lvalue references. For capture by-ref you've forgot to put '&' before values:
return [&values...](){
foo(values...);
};
A small example.
The only diff between the two files:
diff omg.cpp nyan.cpp
6c6
< return [values...]{ foo(values...); };
---
> return [&values...]{ foo(values...); };
Compile the one with by-value capture:
$ g++ -std=c++14 omg.cpp && ./a.out
omg.cpp: In instantiation of ‘bar(T&& ...)::<lambda()> [with T = {int&}]’:
omg.cpp:6:16: required from ‘struct bar(T&& ...) [with T = {int&}]::<lambda()>’
omg.cpp:6:38: required from ‘auto bar(T&& ...) [with T = {int&}]’
omg.cpp:11:16: required from here
omg.cpp:6:25: error: binding reference of type ‘int&’ to ‘const int’ discards qualifiers
return [values...]{ foo(values...); };
~~~^~~~~~~~~~~
omg.cpp:3:6: note: initializing argument 1 of ‘void foo(int&)’
void foo(int &a) { std::cout << a << std::endl; }
^~~
Compile the other:
$ g++ -std=c++14 nyan.cpp && ./a.out
42
314