The impact of C++ function default parameters and variable parameters on program performance

Default parameters are expanded at compile time and do not affect runtime performance; variable parameters will generate runtime overhead and intensive use should be avoided.

The impact of C function default parameters and variable parameters on program performance

Default parameters

Default parameters allow functions to specify default values ​​without passing actual parameters. Default parameters are expanded at compile time so they do not affect the runtime performance of your program.

For example, consider the following function:

int computeAverage(int n, int x = 0) {
  return (n + x) / 2;
}

In this function, x has a default value of 0. If you don't specify x when calling a function, it will use the default value.

Variadic parameters

Variadic parameters allow a function to accept a variable number of arguments. Variable parameters must be the last parameter of the function and are represented by ....

For example, consider the following function:

int sumNumbers(int n, ...) {
  va_list args;
  va_start(args, n);

  int sum = n;
  while (true) {
    int num = va_arg(args, int);
    if (num == 0) {
      break;
    }
    sum += num;
  }

  va_end(args);
  return sum;
}

This function can accept any number of integers and sum them until it encounters 0 (the sentinel value).

Performance impact

• Default parameters: Default parameters do not affect the runtime performance of the program because they are expanded at compile time of.
• Variadic parameters: Variadic parameters incur some runtime overhead because they involve managing a variable number of arguments. This may affect the performance of programs that make intensive use of variadic arguments.

Practical case

Consider the following program, which uses the computeAverage() and sumNumbers() functions:

int main() {
  // 默认参数
  int avg1 = computeAverage(10);
  int avg2 = computeAverage(10, 5);

  // 可变参数
  int sum1 = sumNumbers(10, 20, 30, 0);
  int sum2 = sumNumbers(10, 20, 0);

  return 0;
}

In this program:

• Call the computeAverage() function twice, once with the default parameters and once passing the actual parameters.
• Call the sumNumbers() function twice, passing three numbers once and two numbers once.

The performance of the program will not be affected by the default parameters. However, the use of variadic arguments may incur some overhead, especially if the function is called multiple times and passed a large number of arguments.

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