What are the C++ function calling conventions?

There are four function calling conventions in C: pass by value, pass by pointer, pass by reference and pass by RVO. Passing by value passes a copy of the parameter, passing by pointer passes the address of the parameter, passing by reference passes the reference of the parameter, and passing by RVO moves the contents of the object directly under certain conditions.

C Function calling convention

The function calling convention specifies how parameters are passed during a function call, and when the call returns How to return results. There are four main function calling conventions in C:

1. Pass-by-value

• A copy of the argument is passed to the function .
• The function operates on the copy and does not affect the original value.
• Efficient for basic types (int, float, etc.).

2. Pass-by-pointer

• The address of the parameter is passed to the function.
• Function can point to and modify the original value through a pointer.
• Allows functions to return a variable number of arguments.

3. Pass-by-reference

• The reference (alias) of the parameter is passed to the function.
• Function can point to and modify the original value through reference.
• More efficient than passing by pointer (avoiding pointer dereference).

4. Passing through RVO (return value optimization, return value optimization)

• When a function returns a non-reference object and the object has not been When referenced by any other object, the compiler will perform RVO.
• In RVO, functions move the contents of an object directly into the caller's context, rather than creating and returning a temporary copy.

Practical case

// 通过值传递整数
void func_by_val(int val) {
  val++;  // 不会影响原始值
}

// 通过指针传递整数
void func_by_ptr(int *val) {
  (*val)++;  // 会影响原始值
}

// 通过引用传递整数
void func_by_ref(int &val) {
  val++;  // 会影响原始值
}

int main() {
  int a = 5;

  func_by_val(a);
  std::cout << a << std::endl;  // 输出 5

  func_by_ptr(&a);
  std::cout << a << std::endl;  // 输出 6

  func_by_ref(a);
  std::cout << a << std::endl;  // 输出 7
}

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