This in brackets, what does it mean?

General JUCE discussion
1

I’m beginning to learn Juce and have som knowledge about c++ and I wonder about the code pattern with the this reference in brackets. What do the brackets mean?
frequencySlider.onValueChange = [this] { targetFrequency = frequencySlider.getValue(); };

Found for example in this tutorial:
Sine synth

2

Look up lambdas in C++. That line defines a function which uses the current object (this) inside the function body.

1 Like
3

You are seeing a lambda here. A lambda can be compared to a free function, but instead of a regular function you can declare it inside a function body and make it access variables that are available in this context. To specify what should be accessible to a lambda the so called capture list is used – here the this pointer of a class is captured, allowing you to access member variables and functions of the class in which the lambda is created.

Example:

void printFreeFunction (int value)
{
    std::cout << value << std::endl;
}

class Foo
{
public:
    Foo (const juce::String& n) : name (n)  {}

    void bar()
    {
        // Let's print a number to the console using our free function above
        printFreeFunction (42);

        // We need a printing function that adds the name of this instance to the output, let's declare a
        // lambda for this
        auto printLambda = [this] (int value) 
        {
            std::cout << value << " from " << name.toRawUTF8() << std::endl;
        }

        // We can call our lambda just like a function. But in contrast to the free function above, it has 
        // access to the "name" value of this special class.
        printLambda (42);
    }

private:
    const juce::String name;
}
3 Likes
4

Ok, lambdas I’m a bit familiar with from Python so I see how that works here. Thanks.

However, (comparing a bit with Python) in this example I don’t see why [this] is needed. Isn’t 42 passed as an argument regardless? Perhaps if value was a property of Foo it would make more sense? Or am I missing some detail?

5

In that example, the “this” is needed because name is a member of class Foo, and is used in the lambda. In your example, “this” is needed because frequencySlider is a member of the component in which that code exists.

6

OK, I see. Thanks.