C++ Closure

Introduction

In C++, lambda expression constructs a closure, an unnamed function object capable of capturing variables in scope. It still sounds ambiguous, at least to me. Closure is a general concept in programming that originated from functional programming. When we talk about the closures in C++, they always come with lambda expressions.

In this blog post, we would take a look at an example for C++ lambda expression and closure, learn the difference between lambda expression and closure, and understand the concepts.

Lambda Expression VS Closure

The difference between the concepts of lambda expression and closure is sometimes confusing, since lambda expression and closure were talked about together all the time.

Scott Meyers got a good explanation to this using analogues. “The distinction between a lambda and the corresponding closure is precisely equivalent to the distinction between a class and an instance of the class. A class exists only in source code; it doesn’t exist at runtime. What exists at runtime are objects of the class type. Closures are to lambdas as objects are to classes. This should not be a surprise, because each lambda expression causes a unique class to be generated (during compilation) and also causes an object of that class type–a closure–to be created (at runtime).”

Examples

closure.cpp

#include <iostream>
#include <functional>

std::function<void(void)> closureWrapper1()
{
    int x = 10;
    return [x](){std::cout << "Value in the closure: " << x << std::endl;};
}

std::function<void(void)> closureWrapper2()
{
    int x = 10;
    return [&x](){x += 1; std::cout << "Value in the closure: " << x << std::endl;};
}

int main()
{
    int x = 10;
    auto func0 = [&x](){x += 1; std::cout << "Value in the closure: " << x << std::endl;};
    std::function<void(void)> func1 = closureWrapper1();
    std::function<void(void)> func2 = closureWrapper2();
    func0();
    func0();
    func0();
    std::cout << "-------------------------" << std::endl;
    func1();
    func1();
    func1();
    std::cout << "-------------------------" << std::endl;
    func2();
    func2();
    func2();
}

To compile the program, please run the following command in the terminal.

$ g++ closure.cpp -o closure -std=c++11

The outputs are as follows on my computer.

$ ./closure 
Value in the closure: 11
Value in the closure: 12
Value in the closure: 13
-------------------------
Value in the closure: 10
Value in the closure: 10
Value in the closure: 10
-------------------------
Value in the closure: 32765
Value in the closure: 32766
Value in the closure: 32767

In the above example, func1 and func2 are not closures. Instead, they are std::function wrapper objects that wrapped closures. func0 is a closure, but it should be a copy of the closure created by the lambda expression [&x](){x += 1; std::cout << "Value in the closure: " << x << std::endl;}.

In the func0, it captured the reference to the variable x in the scope of main. Therefore, every time we call func0, the value of x in the scope of main gets increased by 1.

In the func1, it captured the value of the variable x in the scope of closureWrapper1 by making a copy of it. Therefore, every time we call func1, the value of of the closure is always 10. Note that after returning from the ordinary function, the local variables in the ordinary function would be out of scope.

In the func2, it captured the reference to the variable x in the scope of closureWrapper2. The reference “remember” the address of x. However, after returning from the function, the local variable x in the ordinary function would be out of scope. Then the value of reference would be undefined.

Closure Analogs

Function Object (Functor)

Function object overload the operator(). It could capture the values by making a copy of the variables to its member variables. The shortcoming is that for each different function call, regardless of how simple it is, we would have to implement a new class, whereas implementing a lambda expression is faster.

Functions Using Static Variables

We don’t actually like to use static variables in the function, unless it is extremely necessary, because it would confuse the readers. In addition, if you have a lot of function calls, it is likely that we have a lot of static variables which is more undesired.

FAQs

Is Function Object Closure?

No.

According to the definition of closure, “In programming languages, a closure, also lexical closure or function closure, is a technique for implementing lexically scoped name binding in a language with first-class functions”. As C++ does not allow defining functions and objects inside a function, function object does not (always) allow lexical scoping, where with lexical scope, a name always refers to its (more or less) local lexical environment. In our case, x in the closure has always to be mapped to the x in the local scope. In a function object, the member variables are different from the local variables outside the function object, even though they might have the same name.

This might look like a lexical scoping exception for function objects in C++.

#include <iostream>
#include <functional>

double pi = 3.1415926;

class CircleArea
{
public:
    CircleArea()
    {
    }
    double operator() (double r) const
    {
        return pi * r * r;
    }
};

int main()
{
    double r = 1.0;
    CircleArea circleArea;
    double area = circleArea(r);
    std::cout << area << std::endl;
}

However, because we are not allowed to define a class in all the other scopes, class is not considered to support lexical scoping. For example, we could not compile the following program.

#include <iostream>
#include <functional>

int main()
{
    double pi = 3.1415926;
    
    class CircleArea
    {
    public:
        CircleArea()
        {
        }
        double operator() (double r) const
        {
            return pi * r * r;
        }
    };

    double r = 1.0;
    CircleArea circleArea;
    double area = circleArea(r);
    std::cout << area << std::endl;
}

$ g++ closure.cpp -o closure -std=c++17
closure.cpp: In member function ‘double main()::CircleArea::operator()(double) const’:
closure.cpp:16:20: error: use of local variable with automatic storage from containing function
   16 |             return pi * r * r;
      |                    ^~
closure.cpp:6:12: note: ‘double pi’ declared here
    6 |     double pi = 3.1415926;
      |            ^~

Therefore, the only nested functions allowed in C++ are lambda expression.

Conclusion

When we talked about the closures in C++, they are basically referring to the objects that lambda expressions constructed.

References

• Lambdas VS Closures