Should templated functions take lambda arguments by value or by rvalue reference?

Question

GCC 4.7 in C++11 mode is letting me define a function taking a lambda two different ways:

// by value
template
void foo(FunctorT f) { /         


        

Answer 1

This is a good question -- the first part: pass-by-value or use forwarding. I think the second part (having FunctorT& as an argument) has been reasonably answered.

My advice is this: use forwarding only when the function object is known, in advance, to modify values in its closure (or capture list). Best example: std::shuffle. It takes a Uniform Random Number Generator (a function object), and each call to the generator modifies its state. The function object is forwarded into the algorithm.

In every other case, you should prefer to pass by value. This does not prevent you from capturing locals by reference and modifying them within your lambda function. That will work just like you think it should. There should be no overhead for copying, as Dietmar says. Inlining will also apply and references may be optimized out.