On Fri, Apr 9, 2010 at 11:55 AM, Giovanni Piero Deretta <gpderetta@gmail.com> wrote:
On Fri, Apr 9, 2010 at 6:43 PM, Alexey Tkachenko [snip]
Both Lambda and Phoenix pass arguments by reference or by const reference.
IIRC lambda does perfect forwarding up to 3 arguments (i.e. it has overloads for all const and non const combinations up to 3 args).
Phoenix does not: it takes its parameters by non const reference.
Moreover if I write following code it compiles fine: int add(int a, int b) { return a + b; } void bind_expression() { const int i = 5; cout << bind(add, _1, _2)(i, i) << endl; cout << bind(add, _1, _2)((const int&)10, i) << endl; } But if I change second 'bind' to (type cast is removed): cout << bind(add, _1, _2)(10, i) << endl; compilation failes.
For me it's kind of magic. I use boost 1.39 and my compiler - g++ 4.3.1
By explicitly casting to const int&, I think that, for the expression:
bind(add, _1, _2)((const int&)10, i)
T1 is deduced as const in for
template <class T1, class T2> operator()(T1&, T2&)
forming a const reference that can bind to the temporary. I had never realized that the cast would have worked!
For note, I think it is expected for the new Phoenix v3 to use the Functional.Forwarding adapters to fix that bug up to any number (not just 3 or so) of arguments.