c++ - C++11 and the lack of polymorphic lambdas - why?

Question

I've been reviewing the draft version of the C++11 standard. Specifically the section on lambdas, and I am confused as to the reasoning for not introducing polymorphic lambdas.

For example, amongst the 100001 ways polymorphic lambdas could be used, I had hoped we could use code such as the following:

template<typename Container>
void foo(Container c)
{
    for_each(c.begin(), c.end(), [](T& t) { ++t; });
}

What were the reasons:

• Was it that the committee ran out of time?

• That polymorphic lambdas are too hard to implement?

• Or perhaps that they are seen as not being needed by the PTB?

Note: Please remember the example above is not the only one, and it is only provided as a guide to the types of code. Answers that solely concentrate on providing a workaround for the above piece of code will not be considered as valid!

Related sources:

• Lambda expressions and closures for C++ (document number N1968=06-0038)
• Can lambda functions be templated?

Answer 1

The reason we don't have polymorphic lambdas is explained pretty well in this posting.

It has to do with the concepts feature that was pulled from C++11: essentially, polymorphic lambdas are ordinary, unconstrained function templates and we didn't know how to typecheck a concept-constrained template that used an unconstrained template. However, solving that problem turns out to be easy as shown here(dead link), so I don't think there's any obstacle remaining.

The link to cpp-next is dead; the relevant info can be found here