Pointer function

Functions that return pointers

int* array_() {
    
	int* ptr = (int*)malloc(10 * sizeof(int));
	for (int i = 0; i < 10; i++) {
    
		*(ptr + i) = 10;
	}
	return ptr;
}

int main() {
    

	int* temp = array_();
	for (int i = 0; i < 10; i++) {
    
		cout << *(temp + i) << " ";
	}
	cout << endl;

	return 0;
}

10 10 10 10 10 10 10 10 10 10

A function pointer

Declared function pointer

//  Declare a function pointer 
	void (*foo) ();

As shown in the figure , Declaration of function pointer

Be careful , Don't mix up. A function pointer and Pointer function

	int (*fptrl)(int, int);  // Pointer to function 
	int* function(int, int); // A function that returns a pointer type 

We can also use typedef Declare a type definition

typedef int (*funcptr)(int,int);//funcptr  For this type 
funcptr fptr2 = sum;

Calling a function through a function pointer

The function pointer stores the address of the function , Used to point to a function , Different functions can be called by function pointers

But the function pointer must be of the same type as the function it points to , in other words , The return value type and parameter type should be consistent with the number of parameters

// Declared function pointer 
typedef int (*function_ptr)(int, int);

// The function that needs to be pointed to 
int sum(int a, int b) {
    
	return a + b;
}
int main()
{
    
	//fptrl = sum2; Type mismatch , This returns to double type , I also accept two double Parameters of 
	function_ptr fptrl = sum;
	cout << fptrl(2, 3) << endl; // 5

	return 0;
}

In this case , We use a function pointer to call a function , But we didn't add... Before the function &, The function name is similar to the array name , You can directly return the address of the function

int (*fptrl) (int);
int square(int num) {
    
	return num * num;
}

int main()
{
    
	fptrl = square;
	cout << fptrl(2) << endl; //4
	
	return 0;
}

This is an example of calling another function , Understand how functions in memory are called

Pass function pointer

Just declare the function pointer as a function parameter

// Declared function pointer 
typedef int (*fptrOperation)(int, int);

// Add 
int add(int num1, int num2) {
    
	return num1 + num2;
}

// Subtraction 
int sub(int num1, int num2) {
    
	return num1 - num2;
}

// Accept function pointer , And two for calculation int Parameters 
int compute(fptrOperation operation, int a, int b) {
    
	return operation(a, b);// Returns the result of the operation 
}


int main()
{
    
	cout << compute(add, 5, 6);
	cout << compute(sub, 5, 6);


	/*fptrOperation fptr = sum; cout << "the result(+) is: " << compute(add, 10, 5) << endl; cout << "the result(+) is: " << compute(fptr, 10, 5) << endl; cout << "the result(+) is: " << evaluate('+', 10, 5) << endl; fptr = sub; cout << "the result(-) is: " << compute(sub, 10, 5) << endl; cout << "the result(-) is: " << compute(fptr, 10, 5) << endl; cout << "the result(+) is: " << evaluate('-', 10, 5) << endl;*/


	return 0;
}

Return function pointer

typedef int (*fptrOperation)(int, int);

// Add 
int add(int num1, int num2) {
    
	return num1 + num2;
}

// Subtraction 
int sub(int num1, int num2) {
    
	return num1 - num2;
}

// Calculation 
int compute(fptrOperation operation, int a, int b) {
    
	return operation(a, b);
}

// Returns the function pointer according to the character 
fptrOperation select(char opcode) {
    
	switch (opcode) {
    
	case '+': return add;
	case '-': return sub;
	}
}

// Accept characters and call select function , Accept select Function pointer returned , Call the function pointed to by the function pointer 
int evaluate(char opcode, int num1, int num2) {
    
	fptrOperation operation = select(opcode);
	return operation(num1, num2);
}

int main()
{
    
	fptrOperation fptr = add;
	cout << "the result(+) is: " << compute(add, 10, 5) << endl;
	cout << "the result(+) is: " << compute(fptr, 10, 5) << endl;
	cout << "the result(+) is: " << evaluate('+', 10, 5) << endl;

	fptr = sub;
	cout << "the result(-) is: " << compute(sub, 10, 5) << endl;
	cout << "the result(-) is: " << compute(fptr, 10, 5) << endl;
	cout << "the result(+) is: " << evaluate('-', 10, 5) << endl;

	return 0;
}

See here , Did you learn C++ Do you feel a little familiar , That's the smell of polymorphism ~
So , It's important to lay a good foundation ！