Augustus Saunders wrote:
auto x = [](int i){ std::cout << i << std::endl; }; (*static_cast<decltype(x)*>(0))(5);
Right, since without a capture, the implicit this pointer passed to the operator() of the anonymous class is never dereferenced and so just static_casting 0 or null won't actually cause an error.
Unless "Core issue 232" is not resolved, http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232 I think this results in undefined behavior.
Instead, I recommend wrapping lambdas into boost::optional and constructing with in-place factories:
auto x = [](int i){ return i * 100; }; // Store type and value in x
boost::optional<decltype(x)> y; // Get type of x and default construct y y = boost::in_place(x); // Get value of x and lazily construct lambda
I'm not sure I follow here. Let me make a more concrete example and maybe you can fill in the blank:
template <typename LAMBDA> int execute_me(int input) { return (*static_cast<LAMBDA*>(0))(input);
// ok now same thing using optional? boost::optional<LAMBDA> y; return y(input); // ?? obviously not but what? }
The approach with boost::optional cannot do this. In-place factories need a lambda object itself. So maybe you're not interested in this approach. (But, please note that we cannot know the type of lambda expression without actually constructing lambda objects.) Regards, Michel