C++11: Lambda Expressions

Posts in this series
Getting started with C++11
C++11: Initializer lists and range-for statements

So what is a lambda expression?
A lambda expression is way in which to write a function in-line in your code, the typical use case is where you call a function which expects a pointer to another function in order to tailor it to your own needs.  For example, if you had a list of integers (4, 1, 6, 2, 13) and you wanted them sorting, you would typically call a sort method, passing it your list of integers.  Now that sort method may accept a pointer to another function in which you can specify how your list is to be sorted, typically this means that you would have one function defining how to sort in an ascending manner, and another defining a descending manner.  Lets start with an example as to how you would currently do this.

• bool SortAscending(const int x, const int y)
• {
•     return x < y;
• }
• ...
• vector<int> myList = { 4, 1, 6, 2, 13 };
• sort(myList.begin(), myList.end(), SortAscending); // 1, 2, 4, 6, 13

Full example code

So now, if we wanted to sort this in a descending manner then we would need to write a new method to tell the sort algorithm how to sort the values and pass this to the sort method.

This is a really simple function so why do we have to write a whole method for it?  Well using lambda expressions we no longer have to, with the above being written as follows instead.

• vector<int> myList = { 4, 1, 6, 2, 13 };
• sort(myList.begin(), myList.end(), [](int x, int y) -> bool { return x < y; }); // 1, 2, 4, 6, 13

Full example code|

So what did we do here?  Well lets have a look at the lambda expression.

• [](int x, int y) -> bool { return x < y; }

The opening brackets "[]" is a capture list, more on this later.  Next we define the expressions parameter list "(int x, int y)" just as we would if we were defining a normal method.

The next part is the return type "-> bool", this is an optional part of the expression and we could have easily left it out as the compiler can easily determine from the expression body that the return type is a bool; if the return type is not specified and the expression body does not return any value then a return type of void is deduced.  It is useful, however, to specify a return type if you want to explicitly instruct the compiler what the type being returned should be.

Finally there is the expression body which is every between the "{}" braces.

Capture lists
Sometimes when we use lambda expressions we may want to use or modify values from the surrounding code.  Normally we would just pass these in using a parameter, but with lambda expressions we have to provide a parameter list which the receiving method is expecting, in the case of the sort algorithm method, it is expecting an expression which takes two integers and returns a bool.

To access these surrounding values we can use capture lists.  You've already seen a simple use of a capture list in the previous example where we wrote "[]", this tells the compiler that we are not capturing any values for use in the lambda expression.  In this next example we are going to capture an integer variable called "stepCount" which will be incremented each time our lambda expression is used by the sort algorithm, this will tell us how many times the expression was used to completely sort our list.

• int stepCount = 0;
• sort(myList.begin(), myList.end(), [&](int x, int y) -> bool { ++stepCount; return x < y; }); // stepCount is 9

Full example code

The capture list this time is written as "[&]", this instructs the compiler to capture any local variables used in the lambda expression and pass-by-reference.  This allows us to have the sort method update a local variable in our calling code.  Another option for the capture list is "[=]" which tells the compiler to pass-by-value.  If we were to use this in our code however we would get a compiler error as any values passed by value are read-only.

There are other options available for capture lists, these are as follows:

[]	Capture nothing
[&]	Capture variables by reference
[=]	Capture variables by value (make a copy)
[=,&foo]	Capture foo by reference, all other variable by value
[foo]	Only capture foo and do so by value
[this]	Capture the this pointer of the enclosing class

One thing to be careful of here however is that if you return a lambda expression from a method (we'll see how in a second) and you are using a local variable in that method and capturing by reference then you are going to run into problems as the moment you return the expression the local variable you captured as gone out of scope.  Other than that hopefully you can see just how useful these are by now.

Accepting lambda expressions in my own code
One of the great new features in C++11 is the std::function type (and std::bind which I'll cover in a later post) which is a great way for us to start passing around lambda expressions as parameters or return types; in fact it allows us to use lambda expressions and functions.  The structure of the type is std::function<return_type (parameter list)>, so if we wanted to write a method that accepted a lambda expression we could write the following.

• double Sum(const vector<double>& values, function<double (double x)> f)
• {
•     double result = 0.0;
•     for (auto d : values)
•     {
•         result += f(d);
•     }
•     return result;
• }

The "f" parameter is a function which takes a single double parameter and which returns a double value, this is then used in the method body as part of the accumulation process.  The method can then be used as follows.

• double Complex(double x)
• {
•     double result = sin(x * 3.14159265);
•     result -= floor(result);
•     
•     return result;
• }
• int main(int argc, char** argv)
• {
•     vector myValues = { 1.3, 2.1, 7.4, 9.6 };
•     
•     double result1 = Sum(myValues, [](double x) { return x; }); // 20.4
•     double result2 = Sum(myValues, Complex); // 0.597887
• }

Full example code

Here we're calling the "Sum" method twice, the first time with a lambda expression which simply returns the value passed into it so that we sum all of the values.  The second call passes a reference to the "Complex" method, which may not be that complex but illustrates the point about being able to pass standard methods as well as lambda expressions.

Lambda expressions provide an easy and convenient method of providing short functions to other other functions which require thema.  Even if you're not sure if you want to use lambda expressions, making sure that your methods use the std::function type means that you can carry on writing helper functions if you want to but that you or others have the option of using lambda expressions if they want to.

For the next post I should be looking at smart pointers, these give us the benefits of pointers in C++ but also provide better memory management, which can only be a good thing

References
CProgramming.com - Lambda Expressions in C++ - the definitive guide
Bjarne Stroustrup - C++11 FAQ

All code provided in this article is provided under a BSD license.  If you spot an error then please do let me know so that we can make this better for anyone else reading it.

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