1. What is Function Pointer?
In C++, the compiler converts our source code to a transitional code called Object Code. One can see files with ‘.obj’ extension after this translation. After this the linker will merge the translated code and makes the binaries. While we run a program, the binaries load into a memory location called Code Segment. When there is a function, its first instruction set can be located at a certain memory location within this segment. In C++, we can store this starting address in a pointer variable and it is called Function Pointer.
In this example, we will write two functions. One will perform the square of a number and the other one will perform cubic of a number. Then, we will pass these functions as arguments to some other function. On the other hand, The called routine will make a call to these Square & cube by using the passed in function pointers.
2. Code Example
The below example shows how one can pass a function as a parameter to another function. First, go through this code example. Then look at the next section for the explanation of the code flow.
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// TestIt.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include "conio.h" //Sample 01: Function that performs Square long Square_it(long x) { return (x * x); } //Sample 02: Function that perform Cube long Cube_it(long x) { return (x * x * x); } //Sample 03: Function that Take function pointer long summation(long summationFn(long), int n) { int sum = 0; for(int i=1; i<=n; i++) sum = sum + summationFn(i); return sum; } int _tmain(int argc, _TCHAR* argv[]) { int total = 0; //Sample 04: Calculate Square summation upto 4 total = summation(Square_it, 4); printf("Square Summation upto 4= %ld\r\n", total); //Sample 05: Calculate Cubic Summation upto 4 total = summation(Cube_it, 4); printf("Cube Summation upto 4 = %ld\r\n", total); getch(); return 0; } |
3. Code Explanation
- The functions Square_it , Cube_it will calculate the square and cubic of the provided numbers.
- The summation will take the first parameter as a function pointer. Here, the function pointer passed as a parameter is supposed to fall under certain signature. Means, it should take long as a parameter and return a long value to the caller. The second parameter for the summation function tells when we want to end the summation series. Have a look at the syntax of passing the function pointer as a parameter long summationFn(long). Here, summationFn is the function pointer name.
- The summation function will call the passed-in function for n number of times. As the parameter is a pointer to a function which can take long and return a long, it can be any function adhering to that rule. In our case, it can be Square_it or Cube_it .
- In the program main, we called the summation function twice. The first time, we passed the address of the Square_it function. And the second time, we passed the Cube_it function as the function pointer. The result we get in the main function is the Square summation from 1 to 4 and cubic summation from 1 to 4.
The screenshot below shows the program output:
Categories: C++
Tags: Function Pointer, Pointer to Function
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