The relationship between C++ function parameter passing methods and variable parameter functions

Function parameter passing methods include value passing, reference passing and pointer passing. Variable parameter functions can only pass in parameters through pointer passing, because the function needs to know the address of the variable parameter part. For example, the sum() function uses ... to accept an unknown number of arguments, and then uses the va_arg() macro to get the value of the variadic argument.

C The relationship between function parameter passing method and variable parameter function

In C, function parameters can be passed by value, The function is passed by reference or pointer. These different passing methods have a significant impact on the behavior of parameters within the function, and also affect the use of variadic functions.

Function parameter passing method

• # Value passing: A copy of the parameter is passed into the function, and any modification to the copy will not influence external variables.
• Pass by reference: The address of the parameter is passed into the function, and modifications to the parameters will directly affect external variables.
• Pointer passing: The address of the parameter is passed into the function, and the pointer can be used to modify external variables or access members of external variables.

Variable parameter function

Variadic parameter function allows the function to accept an unknown number of parameters. They use the special syntax ... to indicate the variadic part.

Function declaration:

int sum(int count, ...);

Function call:

int result = sum(3, 1, 2, 3);

The relationship between transfer method and variable parameter function

For variable Parametric functions can only use pointers to pass parameters, because the function must know the address of the variable parameter part. Therefore, the parameters of a variadic function are always passed into the function by pointer passing.

Practical case

The following code shows how to use the variable parameter function to calculate the sum of numbers:

#include <cstdarg>
#include <iostream>

int sum(int count, ...) {
  va_list args; // 可变参数列表
  va_start(args, count); // 初始化可变参数列表，count 为固定参数

  int sum = 0;
  for (int i = 0; i < count; i++) {
    sum += va_arg(args, int); // 获取可变参数，将其转换为 int
  }

  va_end(args); // 清理可变参数列表

  return sum;
}

int main() {
  // 调用可变参数函数
  int result = sum(3, 1, 2, 3);
  std::cout << result << std::endl; // 输出结果
}

In this code, sum () The function uses pointers to pass variable parameters ..., and uses the va_arg() macro to obtain the value of the variable parameters.

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