About function pointer in C

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“About C function pointer in C” expressed in English.

Forword

I have coded by using the functional programming in JavaScript and Java.I also have learned the C function pointer which can also be used to program functionally.However,I have seldom or never used it.The article will be writen to interpret the function pointer in C.

Music

Not dyed by He Tu

Body

There are two main themes in the part:

  • C function pointer
  • left-value & right-value

How to use C function pointer

The function pointer can be declared in this way:

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typedef int (*func_ptr)(int,int);

In this way,we can declare a function pointer which points to a class of functions using two int parameters and int return-values.

While in JavaScript,We need not be careful about it.

In the underlying architecture,the reasons why a function pointer can only point to a class functions which have the same parameters and return-value are as the following:

  • the compiler need to ensure the number and type of parameters to allocate the memory space for it,and to perform the correct operation.
  • the compiler need to know the return-value type of the function.

Thus,the function pointer must be designated the return-value type and the parameter type.It’s the same as the data pointer which has definite specification.

An example

We declare the function pointer which is as follows:

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typedef int(*func_ptr)(int,int);

Then we define a function named maximum:

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int maximum(int a,int b){
    return a>b?a:b;
}

Then in the main-function,we use the function pointer in this way:

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int main(){
    func_ptr p=&maximum;// '&' can be ignored.
    //func_ptr p=max;
    int a=1,b=2,c=3,d=4;
    printf("Maximum of four numbers is %d!\n",p(a,p(b,p(c,d))));
    return 0;
}

// As what you think,the result is 4.

Write callback function by using functional programming in C

You maybe have used the callback function in C GUI programming such as Win32 API and MFC.But how does it make it?

Callback can be implemented by using function pointer.And function pointer is always used as callback function.

We have declared the function pointer func_ptr before,and maximum function.Then we declare another function and implement a callback function:

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typedef int(*func_ptr)(int,int);

int maximum(int a,int b){
    return a>b?a:b;
}

int minimum(int a,int b){
    return a<b?a:b;
}

int max_min(int a,int b,int c,int d,func_ptr max_min_func_ptr){

    //Do something here!

    //As the following: revoke the callback function.
    return max_min_func_ptr(a,max_min_func_ptr(b,max_min_func_ptr(c,d)));
}

int main(){
    func_ptr pmax = maximum;
    func_ptr pmin = minimum;
    int a=1,b=2,c=3,d=4;
    printf("Maximum of four numbers is %d!\n",max_min(a,b,c,d,pmax));
    printf("Minimum of four numbers is %d!\n",max_min(a,b,c,d,pmin));
}

Result:

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Maximum of four numbers is 4!
Minimum of four numbers is 1!
Press any key to continue . . .

In this way,we use these two functions named maximum and minimum as the callback functions.

What is left-value and right-value

The crucial different between both in C++:

  • left-value: Can get its address in the memory.

    int a,int a[2],char a,struct {} a,function a…

  • right-value: Cannot get its address.

    2,3.1…

Instead of being assignable or not

As before,we can conclude that the function designator(function name) is a left-value.Of course,that is a left-value in C++.
While in C,it’s neither a left-value nor a right-value.

Conclusion

In this article,we are familiar with c function point,callback and lr values.

That’s all for the time being.Thanks for your reading!

See you later.