Some days ago I wrote my first lambda expression in C++.
int n=0;
std::for_each( begin, end, [[&n]](std::uint8_t& a){a=n++;});
If you are back at the C++85 age, this could look like any other alien language from a Star Trek episode. If you are updated at the more recent C++98, the syntax would look funny at minimum.
At least, that is what it looked to me before starting to get some information about the new standard (you can find a brief yet comprehensive recap of what changed in C++ on this specific page of Bjarne’s website).
You should read the code above as follows. The square brackets defines the intro for the lambda function. Within the square brackets you should put what the lambda function should capture of the environment.
In the code, I stated that n should be captured by reference (&). I could have replaced the ampersand if I wanted to have the capture happen by value. Or I could have put nothing should I wanted the lambda function to capture everything.
Next the argument comes, not different from standard function. Eventually the function body.
Once you get this quick start, you will easily decode as a way to fill a range with increasing integers. Take a breath, get another look at the code, ok, now it should make sense.
Well, I’m quite with Stroustrup when he says that he has mixed feelings about lambda functions (I am looking for the quote, but I don’t have it handy). For simple function lambdas are a godsend. On the other hand, lambdas can yield a terrific potential of hiding mechanism and causing major headaches should they escape your control.
If you compare the line above with the code you have to write previously, it is obvious that lambda expressions are a giant leap forward.
In the C++98 days you ought to write –
class DisposableNameForClass
{
public:
DisposableNameForClass() : n( 0 ) {}
void operator() ( int& a ) { a = n++; }
private:
int n;
};
//..
DisposableNameForClass disposableNameForInstance;
std::foreach( begin, end, disposableNameForInstance );
And that is very uncomfortable and inconvenient. By looking at code like this it easy to question whether it makes sense to use the std::for_each at all rather than roll your own code.
But, look at the traditional code
int n=0;
while( begin != end )
{
*begin++ = n++;
}
This code is pretty clear to anyone has a minimum familiarity with C or one of the derived language (yes, there is the dereference operator which involves pointers, but shouldn’t pose real obstacles to comprehension).
Is it error prone? Possibly as any code longer than 0 bytes. std::for_each saves you at least two errors – messing up the loop termination condition (e.g. forgetting the ! from the comparison, or comparing for equality rather than inequality) and missing the increment of the iterator (this happened to me at least once).
These reasons may not be enough to recommend std::for_each in C++98, but it is hard to argue against if you can use the lambda functions.
