代码中*((T**)m_ptr)及*((T**)ptr)=m_address的用法问询

Hey there! Let's break down this syntax clearly—it's a common pattern in low-level C/C++ code, especially when working with generic or pointer-manipulation logic. Let's split it into parts to make it easier:

Step 1: The Type Cast ((T**)ptr) #

First, (T**) is a type cast that tells the compiler: "Treat the pointer ptr as a pointer-to-a-pointer-to-type-T" (a double pointer).

For example, if ptr was originally a void* (a generic pointer type), this cast converts it to T** so the compiler knows we're dealing with a memory location that stores a T* pointer.

Step 2: The Dereference *((T**)ptr) #

Adding the leading * dereferences that double pointer. So:

• ((T**)ptr) = a pointer to a T*
• *((T**)ptr) = the actual T* pointer stored at that memory address

Now let's apply this to the two code snippets you saw:

Case 1: Reading a Pointer Value with *((T**)m_ptr) #

This line reads the T* pointer that's stored at the memory location pointed to by m_ptr.

Imagine m_ptr points to a variable of type T*—this syntax lets you fetch that variable's value (the address it points to) even if m_ptr was originally a generic type like void*.

Case 2: Writing a Pointer Value with *((T**)ptr) = m_address #

This line writes the T* pointer m_address into the memory location pointed to by ptr.

This is super common in generic functions where you need to modify a pointer from outside the function. For example, if you have a function that takes a void* (to work with any pointer type), you can cast it to T**, dereference it, and assign a new pointer value to the original pointer variable.

Example in Action #

Here's a concrete code example to make it click:

#include <stdio.h>

// Generic function to set a pointer (using void*)
void update_pointer(void* target_ptr, void* new_address) {
    // Treat target_ptr as a pointer-to-int*, then assign new_address (cast to int*)
    *((int**)target_ptr) = (int*)new_address;
}

int main() {
    int my_num = 123;
    int* my_ptr = NULL; // Start with a null pointer

    // Pass the address of my_ptr (which is int**) as void*
    update_pointer(&my_ptr, &my_num);

    // Now my_ptr points to my_num!
    printf("Value: %d\n", *my_ptr); // Outputs 123
    return 0;
}

In short, this syntax is a way to safely (as safely as type casting gets in C/C++) read or write a pointer value when you're working with a generic pointer type (like void*) and need to interact with a double pointer under the hood.

内容的提问来源于stack exchange，提问作者Bill Lumbert