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I'm updating some code and while I was working in a header, I came across the following line.
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class HereIsMyClass;
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That's it. It's just one line that precedes another, longer class definition. HereIsMyClass is in fact another class somewhere else, but I don't understand why this line is written here. What does it do?
784
a class describes the contents of the objects that belong to it: it describes an aggregate of data fields (called instance variables), and defines the operations (called methods). object: an object is an element (or instance) of a class; objects have the behaviors of their class.
C Classes A class consists of an instance type and a class object: An instance type is a struct containing variable members called instance variables and function members called instance methods. A variable of the instance type is called an instance.
Summary: The most Pythonic way to define a function in a single line is to (1) create an anonymous lambda function and (2) assign the function object to a variable name. You can then call the function by name just like any other regularly-defined function.
It starts with the keyword lambda , followed by a comma-separated list of zero or more arguments, followed by the colon and the return expression. For example, lambda x, y: x+y calculates the sum of the two argument values x+y in one line of Python code.
This line in C++ is a forward declaration. It's stating that at some point in the future a class named HereIsMyClass will likely exist. It allows for you to use a class in a declaration before it's completely defined.
It's helpful for both breaking up circularly dependent classes and header file management.
For example
class HereIsMyClass;
class Foo {
void Bar(HereIsMyClass* p1) ...
};
class HereIsMyClass {
void Function(Foo f1);
}
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