Function pointer and callback function

​ Function names can be treated as constants .

It holds the memory address of the function , The memory address of the function stores the location where the function starts execution ,

Use the pointer to save the address of the function . A pointer to is called a function .

Through function pointers , It can flexibly call various forms （ parameter list ） identical , But functions with different functions .

This greatly increases the flexibility of the code .

For example, define three functions with the same formal parameters and return values , Calculate the two numbers separately and add them , Subtracting the , Multiply ​

​ stay main Function , Define two integer values a,b.

There are two definitions int Parameters , Function pointer whose return value is an integer func_ptr, Point to the three functions defined above .​

​ We call the original function directly , And call the function pointer to get the calculation result , You can find ：

1） func_ptr The stored value is equal to the address in the function name .

2） The result of calling the function directly , The same result as calling a function through a function pointer .

  Like other types of variables , Function pointers can also be passed to functions as function parameters .

The function name itself is the function pointer , Can be passed as an argument to compute_func function .​

The result is a normal call .

​ take sum,difference,product Assign as a parameter to compute_func This process is called the registration of callback functions .

compute_func Call in function sum,differenct,product The process is called the call of callback function .​

 

​ Application scenarios ：

There are two roles , They are the developers of a function , And the user of the function .

The normal scenario is that the user directly calls the function developed by the developer , Just get the return result .

However, some logic of functional functions developed by developers depends on the behavior of users , Depending on the user's behavior , Make different logical processes . At this time, you need to use the callback function .

A hole will be reserved for the general function functions that developers will develop （ Function pointer parameter ）. When using this function , The user needs to pass a function pointer to the function , The function pointed to by this function pointer is developed by the user , When the function is running , The process of calling the user developed function through the passed function pointer is a callback process .

The function developed by the user , It's called a callback function .​

​ Based on the above theory , We as users , You can create your own callback function . As a parameter , Pass to compute_func.

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​ Generally speaking , The developer and the user are not the same person , And the functions developed by developers will be more complex ,

in addition , Whatever the callback function does , As long as the callback function passed in by the user is consistent with the formal parameters of the function pointer parameters .​