Analysis of memory management of default parameters and variable parameters of C++ functions

Memory management of default parameters and variable parameters: Default parameters: Allocate memory in the function stack frame, and the size is the number of bytes of its type. Variable parameters: Allocate memory at the end of the stack frame. The size is determined by the number of variable parameters: sizeof(void) (number of parameters passed in 1)

C function default parameters Analysis of memory management of variable parameters

The function parameter passing mechanism in C involves value copying or reference, which will affect memory management. This article will provide an in-depth analysis of the memory management behavior of default parameters and variable parameters.

Default parameters

Default parameters are specified when the function is defined and are used to provide default values ​​when no actual parameters are passed. They are expanded at compile time and their memory allocation occurs in the function stack frame. For example:

void myFunction(int x = 10);

When the function is called, if the x parameter is not passed, the default value 10 is used. The default parameter's memory allocation size is the size of its type.

Variable parameters

Variable parameters allow a function to accept an indefinite number of parameters. They are represented using ... and are located at the end of the parameter list. Variable arguments are unwound at runtime, and their memory allocation occurs at the end of the stack frame. For example:

void myFunction(int x, ...);

When dealing with variadic parameters, the function creates a variadic parameter list object that stores an array of pointers pointing to the memory addresses of the actual parameters. The memory allocation size of the variable parameter object is sizeof(void *) * (number of parameters passed in 1).

The following example shows the memory management behavior of default parameters and variadic parameters:

#include <iostream>

void withDefault(int x = 10) {
    std::cout << "x in 'withDefault' is: " << x << std::endl;
}

void withEllipsis(int x, ...) {
    std::va_list args;
    va_start(args, x);
    
    int sum = x;
    int arg;
    while (va_arg(args, int) != NULL) { // 获取可变参数并累加
        arg = va_arg(args, int);
        sum += arg;
    }
    
    va_end(args);
    std::cout << "Sum of all arguments in 'withEllipsis' is: " << sum << std::endl;
}

int main() {
    withDefault();
    withEllipsis(1, 2, 3, 4, 5, NULL);
    return 0;
}

Output:

x in 'withDefault' is: 10
Sum of all arguments in 'withEllipsis' is: 15

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