I. Wrong practice of exchanging two data values

Error one:

void swap(int a, int b){int temp = a;a = b;b = temp;}int main(){int x=1;int y=2;swap(x,y);}

Error reason:
mainWhen the function calls the swap function, the swap function allocates two spaces a and b to store the parameters of the main function. After the swap function is executed, the two values ​​of a and b in the stack space allocated in the swap are exchanged.But when the swap function returns to the main function after execution, the two variables a and b are released, and the x and y in the main function have not changed.
Error two:

void swap(int* a, int* b){int* temp = a;a = b;b = temp;}int main(){int x=1;int y=2;swap(&x,&y);}

Error reason:
GoWhen the function calls the swap function, the swap function allocates two pointer variables a and b to store the addresses of x and y. After the swap function runs, the addresses of the two pointer variables are exchanged, a points to y, and b points to x.After returning to the main function, the two pointer variables a and b are released, and the x and y of the main function remain unchanged.

Second, the exchange of two variables is worth the correct practice

Swap constant values:

void swap(int* a, int* b){int temp = *a;*a = *b;*b = temp;}int main(){int x=1;int y=2;swap(&x,&y);}

When the main function calls the swap function, the swap function allocates two pointer variablesa and b point to x and y respectively. The swap function directly accesses the value pointed to by the pointer through *p and exchanges them. When the swap function returns to the main function, the two pointer variables a and b are released, but the x,y is changed.

Swap pointer variable values:

void swap(int** c, int** d){int* temp = *c;*c = *d;*d = temp;}int main(){int x = 1;int y = 2;int* a = &x;int* b = &y;swap(&a, &b);}

When the main function calls the swap function, two pointers are allocated in the swap functionThe variables c and d respectively point to the pointer variables a and b in the main function. When the swap function is executed, the two pointer variables a and b are manipulated through the two pointer variables c and d to achieve the purpose of exchanging the two pointer variables.Only the addresses stored in the pointer variables a and b will be changed, and the values ​​of the constants x and y will not be changed.

void swap(int* c, int* d){int temp = *c;*c = *d;*d = temp;}int main(){int x = 1;int y = 2;int* a = &x;int* b = &y;swap(a, b);}

When the main function calls the swap function, two pointers are allocatedThe variables c and d are used to store the values ​​of the pointer variables a and b, that is, c and d point directly to the variable values, and the variable values ​​are exchanged through *P, so in the end, the variables x and y will change, while the addresses stored in a and b are notwill change.

Summary

The sub-function needs to operate on the variables of the main function. The incoming parameter in the sub-function is the address of the main function variable. If the main function is an ordinary variable, the incoming parameter should be a pointer. If the main function is a pointer variable, the incoming parameter should be a second-level pointer, which is pushed in in turn.Then the sub-function uses the pointer to change the value of the variable of the main function. Do not directly operate the pointer in the sub-function, but operate the pointed variable through *p.The last case is similar to exchanging constant values, in that it changes the value of the constant without changing the pointer to the constant in the main function.