What does const lambda mean?

Question

#include <iostream>
int foo(int i)
{
    const auto a = [&i](){ i = 7; return i * i; };
    a();
    return i;
}
int main()
{
    std::cout << foo(42) << std::endl;
    return 0;
}

This compiles( g++ -std=c++11 -Wall -Wextra -Wpedantic main.cpp ) and returns 49. Which is surprising to me, because by declaring a to be a constant object, I would have expected i to be referenced as const int&. It clearly isn't, why?

Answer 1

[&i](){ i = 7; return i * i; }

is mainly equivalent to

class Lambda
{
public:
    Lambda(int& arg_i) : i(arg_i) {}

    auto operator() () const { i = 7; return i * i;}
private:
    int& i;
};

And so then you have:

const Lambda a(i);
a();

And the const Lambda won't promote its member to const int& i; but int& const i; which is equivalent to int& i;.