Why closure types / lambdas have no default constructor in C++

Question

What is the reason why lambdas have no default constructor? Is there any technical reason behind that, or is it a pure design decision?

Answer 1

Lambdas in C++ are a syntactic convenience to allow the programmer to avoid declaring a functor using traditional syntax like

struct my_functor
{
bool operator()(Object &) { /* do something */ return false; }
}

because writing functors using this syntax is considered too long winded for simple functions.

Lambdas offer parameter capture options e.g [=], [&] etc. , to save you declaring variables and initializing them, like you might do in the constructor of a functor object.

So Lambdas don't expose any constructor syntax to you by design.

There is no technical reason a lambda function could not expose a constructor, however if it were to do so it would stop mimicking the design concept it is named for.

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With thanks to the commentators under the question.