Detailed explanation of C++ function parameters: the essence and precautions of the outgoing mechanism

C There are two ways to pass function parameters: call by value (does not affect actual parameters) and call by reference (affects actual parameters). Passing out parameters is achieved by passing a reference or pointer, and the function can pass the value to the caller by modifying the variable pointed to by the parameter reference or pointer. Please note when using: The outgoing parameters must be clearly declared, can only correspond to one actual parameter, and cannot point to local variables within the function. When calling by passing a pointer, be careful to avoid wild pointers.

Detailed explanation of function parameters in C: The essence and precautions of the outgoing mechanism

The essence of the outgoing mechanism

The function parameters in C There are two main transfer methods: Call by value and Call by reference.

• Call by value: Pass a copy of the actual parameters to the function parameters. Modifications to the parameters within the function will not affect the actual parameters.
• Call by reference: Pass a reference of the actual parameter to the function parameter. Modifications to the parameter within the function will directly affect the actual parameter.

Implementation of outgoing parameters

Outgoing parameters refers to the function passing the value to the caller through parameters. In a pass-by-reference call, the function can be passed out by modifying the variable pointed to by the parameter reference.

In C, you can use the reference (&) symbol or the pointer (*) symbol to implement outgoing parameters:

// 传引用调用
void Swap(int &a, int &b) {
  int temp = a;
  a = b;
  b = temp;
}

// 传指针调用
void Increment(int *ptr) {
  (*ptr)++;
}

Actual case

Calling by reference

int main() {
  int x = 1, y = 2;
  Swap(x, y); // 传引用调用
  
  // x 和 y 的值已经交换
  cout << "x: " << x << ", y: " << y << endl;
  return 0;
}

Calling by pointer

int main() {
  int n = 5;
  // 获取 n 的地址
  int *ptr = &n;

  // 通过指针修改 n 的值
  Increment(ptr);

  // n 的值已增加 1
  cout << "n: " << n << endl;
  return 0;
}

Notes

• Outgoing parameters must be explicitly declared. In function declarations and definitions, outgoing parameters need to be declared using reference or pointer notation.
• The outgoing parameter can only correspond to one actual parameter. If a function has multiple outgoing parameters, each parameter must correspond to a separate actual parameter.
• Outgoing parameters cannot point to local variables within the function. The life cycle of local variables is the same as that of the function. Once the function ends, the local variable will be destroyed and the reference or pointer pointing to it will become invalid.
• Use pointer-passing calls with caution. Pointers are prone to pointing to wild pointers, so you need to ensure that the pointer always points to a valid address.

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