unit testing C++11 closures

Question

Is there any precedence for doing unit testing on C++ closures?

Functions that I write usually start out as closures defined near their point of use, and then (maybe) graduate to full functions later. This is nice for keeping interfaces clean and makes it easier to read the code in a linear fashion, but it undermines writing unit tests.

Are there any tricks or C++ unit testing frameworks that can handle, say, some little function for computing some geometry that is defined as a closure within my main()?

Answer 1

I would think you should be testing functions, not lambda functions. If a function contains lambda functions then they are implementation details. If you are reusing lambda functions by creating them as variables, then those are easily unit tested as functions.

Eg.

auto lambda = [](/* params */){/* stuff */}; // this can be unit tested

void func() // this can be unit tested
{
    // the lambda is an implementation detail of the function
    sort(/* stuff */, [](/* params */){/* stuff */}); 
}

Answer 2

TL;DR: No.

In order to unit test the closure, you would have to give it a name that you can refer to, by assigning it to a variable.

If it is complicated enough to unit test on its own, you should extract a method and test that instead.

Short of that, you can always unit test the closure indirectly, through the method or function which contains it.

Answer 3

In short, no. but..

You can test the code that use the closures. The fact that closures are embedded into source code and you don't have any reflection mechanism prevents you from unit-testing them (however you have many ways to test stuff, not only unit tests), however usually the code using closures is more compact so as long we test the whole block using the closures it is fine using them. I tend to write closure that are only few lines of code, infact you could actually create a function (called by the closure) and unit test the function itself ;).

int function(MyClass *){ // unit test here

}

//...
void MyClass::method(){  // ... and unit test method
    auto f = [this] () { return function(this);};
    applyFunctorOnCollection(f);
}