Parameter passing mode of C language (int x) (int *x) (int & x)

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C There are three ways of transmitting parameters in language

1. Transfer value （int x）

2. Address （int *x）

3. The reference （int &x）

summary

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C There are three ways of transmitting parameters in language

1. Transfer value （int x）

void test(int x){
    
}

Is to pass the value of your variable
Formal parameters to function , In fact, a new formal parameter is generated with the value of the variable , Therefore, the change of parameters in the function will not affect the value of variables outside the function .

Be careful ： Formal and actual parameters are never a variable

The most classic example is to exchange the values of two numbers ,  When a function is called , When an argument is passed to a formal parameter , The formal parameter will be a temporary copy of the argument , Therefore, the modification of formal parameters does not affect the arguments , If you pass parameters in this way , Just exchange the values of formal parameters , Will not really exchange the values of two numbers

void exchange(int a, int b)
{
    
    int t;// Define intermediate variables 
    t = a;
    a = b;
    b = temp;
    
}
int main(int argc, char const *argv[])
{
    int a = 3;
    int b = 5;
    exchange(a, b);
    printf("----------------------\n");
    printf("a=%d\n", a);
    printf("b=%d\n", b);
    return 0;
}

2. Address （int *x）

Is to pass the address of the variable to the pointer of the formal parameter in the function , Make the pointer point to the address of the real variable , Because the change of the content of the address indicated by the pointer can be reflected outside the function , That is, it can change the value of variables outside the function .

void exchange_3(int *p, int *q)
{
    int t;
    // If you want to swap *p and *q Value , be t Must be defined as int, Cannot be defined as int*, Otherwise syntax error 
    t = *p; //p The data type of is int*,*p The data type of is int
    *p = *q;
    *q = t;
}
int main(int argc, char const *argv[])
{
    int a = 3;
    int b = 5;
    exchange_3(&a, &b);
    printf("----------------------\n");
    printf("a=%d\n", a);
    printf("b=%d\n", b);
    return 0;
}

But here's the thing , The size of pointer variables always only accounts for 4 Bytes , So if you pass it with a pointer , Will be faster , It's more efficient .

3. The reference （int &x）

Quote actually Is an alias of a variable , Has the same memory space as this variable . 

Arguments pass variables to formal parameter references , amount to A formal parameter is an alias for an argument variable , The modification of formal parameters is to modify the arguments directly .

Objects are often used as members of the referenced class , Greatly improve the efficiency of the code

For example, when we initialize a stack

int IninStack(SqStack &S)
{
    S.base = (int *)malloc(STACKSIZEINIT * sizeof(int));
    if (!S.base)
    {
        exit(-1); // memory allocation failed , Exit procedure 
    }
    S.top = S.base; // The top stack pointer and the bottom stack pointer refer to the same storage unit 
    S.stacksize = STACKSIZEINIT;
    return 1;
}

summary

Declare a reference , It's not a new variable , It only means that the reference name is an alias of the target variable name , It is not a data type in itself , Therefore, the reference itself does not occupy the storage unit , The system also does not allocate storage units to references . so ： Address a reference , It is to find the address of the target variable .&ra And &a equal .

Reference passing can be regarded as based on value passing , Add a... Before the formal parameter variables defined and declared by the function &, Other uses are exactly the same as value passing , Therefore, it can be seen that reference passing is more convenient