Understanding Multiple Levels of Pointer Dereferences
Pointers are powerful tools in programming languages that allow programmers to access data indirectly. However, in certain situations, it may be necessary to use multiple levels of pointer references, known as double pointers or triple pointers. Let's explore when it makes sense to utilize these complex structures.
Consider the following code snippet:
char * * *ptr;
This triple pointer can be interpreted as follows:
- The first asterisk (*) indicates a pointer to a pointer to a char.
- The second asterisk (*) indicates a pointer to a pointer to a char pointer.
- The third asterisk (*) indicates a pointer to a pointer to a char pointer.
This means that ptr is pointing to a memory address that contains an address of another memory address, which in turn contains an address of a character variable.
When to Use Multiple Levels of Pointers:
In general, it is not common to use triple pointers or even double pointers. However, there are rare situations where they can be beneficial. One such scenario is when working with arrays of structures or objects that contain pointers.
For instance, let's say we have a struct called invocation that represents a shell command invocation:
struct invocation {
char *command;
char *path;
char **env;
...
};
In this struct, env is a pointer to an array of environment variables. Each element of this array is a pointer to a string representing the variable name and value.
To process the environment variables in all invocations, we could create an array of pointers to these variables and pass it to a function:
void browse_env(size_t envc, char ***env_list);
In this example, env_list is a triple pointer because it is a pointer to an array of pointers to arrays of pointers to characters. By dereferencing env_list, we can access the list of environment variables for each invocation.
Conclusion:
While multiple levels of pointer dereferences are rarely used, they can be valuable in specific scenarios. When working with arrays of structures or objects that themselves contain pointers, triple pointers or even double pointers can provide the necessary level of indirection to access the underlying data.
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