Detailed explanation of C++ function calling mechanism

The function calling mechanism in C involves passing arguments to a function and executing its code, returning the result if one exists. There are two ways to pass parameters: pass by value (modifications are made inside the function) and pass by reference (modifications are reflected in the caller). In pass-by-value, value modifications within a function do not affect the original value (like printValue), whereas modifications in pass-by-reference do affect the original value (like printReference).

Detailed explanation of C function calling mechanism

Introduction

In C, functions A call is a mechanism that allows a program to perform a specific task by passing parameters. Function calls involve the following steps:

1. Passing parameters.
2. Transfer control to the function body.
3. Execute the code in the function body.
4. Return the return value to the caller (if the function has a return value).
5. Restore control to the caller.

Value passing and reference passing

There are two parameter passing mechanisms in C:

• Value passing: Parameters are passed to functions by value. Any modifications made to parameters within a function are not reflected in the original values ​​passed in the caller.
• Pass by reference: Parameters are passed to the function by reference. Any modifications made to parameters within a function are reflected in the original values ​​passed in the caller.

Call function by value

void printValue(int value) {
  value++; // 对 value 的修改不会影响调用者传递的原始值
}

int main() {
  int num = 10;
  printValue(num);
  cout << num << endl; // 输出 10
}

Call function by reference

void printReference(int &value) {
  value++; // 对 value 的修改会影响调用者传递的原始值
}

int main() {
  int num = 10;
  printReference(num);
  cout << num << endl; // 输出 11
}

Practical case

Suppose we have a function that finds the greatest common divisor (GCD). The pseudo code is as follows:

int gcd(int a, int b) {
  if (b == 0) {
    return a;
  }

  return gcd(b, a % b);
}

Call the gcd function by value:

int main() {
  int a = 10, b = 12;
  int gcdValue = gcd(a, b);
  cout << "GCD: " << gcdValue << endl; // 输出 2
}

In this case, the original values ​​of a and b will not be affected by the parameter modification in the gcd function.

Calling the gcd function by reference:

int main() {
  int a = 10, b = 12;
  gcd(a, b);
  cout << "GCD: " << a << endl; // 输出 2
}

By passing by reference, the function can modify the value of a. Therefore, the caller receives the GCD value and stores it in a.

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