Ampersands in C++


Author: yifei / Created: July 16, 2017, 9:29 a.m. / Modified: July 16, 2017, 5:30 p.m. / Edit

Ampersands in C++

For the first time I'm working with a C++ program, I'm suprised by so many &s in there, after about one year of learning, I got a summary here.

Starting from C

C language has only one way to pass variable to function calls -- pass value. And there are 2 situations where passing value is not perfect.

int add (int a, int b) {
    return a + b;
}

and use it by calling:

int a = 1, b = 2, c;
c = add(1, 2);

when you need to modify you variable inside the function you are calling, or pass-by-value is too expensive, you will need to use pointers.

void swap (int * a, int * b) {
    int temp = *a;
    *a = *b;1
    *b = temp;
}

and use it by calling:

int a = 1, b = 2;
swap(&a, &b);

For some huge struct:

void do_something (BigStruct * big_struct) {
    // do something on big_struct
}

and use it by calling:

BigStruct big;
BigStruct * big_ptr = &big;
do_something(big_ptr); // or do_something(&big)

We can see the usages are:

Pointers are sharp knives, use it well or you may cut yourself. Pass by reference to the rescue!


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