Nooby pointer question
Author
Suddenly get a little big confused about pointers.
Here are my questions:
assume i defined a class A. Are the following all correct? And what's the difference?
1>
A* a = new A();
delete a;
2>
A* a=null; int num = 10;
a = new A[num];
delete a[];
3>
A* a[10]; int num = 10;
for(int i=0;i<num;i++)
a = new A();
For 3> can I use something like A** a and according to 'num', dynamically create the array?
Apart from the fact that "null" isn't a valid value, yes they are all correct. However, #3 doesn't release the memory it allocates (you would need to loop over and delete each instance).
You can have a dynamically created array of pointers:
Using the double pointer syntax:
You can have a dynamically created array of pointers:
std::vector<A *> array;for(int i = 0 ; i < 10 ; ++i) { array.push_back(new A());}// ...for(int i = 0 ; i < array.size() ; ++i) { delete array;}Using the double pointer syntax:
int count = rand() % 1000;A **array = new A*[count];for(int i = 0 ; i < count ; ++i) { array = new A();}// ...for(int i = 0 ; i < count ; ++i) { delete array;}delete [] array;
Author
Quote:Original post by rip-off
Apart from the fact that "null" isn't a valid value, yes they are all correct. However, #3 doesn't release the memory it allocates (you would need to loop over and delete each instance).
You can have a dynamically created array of pointers:
*** Source Snippet Removed ***
Using the double pointer syntax:
*** Source Snippet Removed ***
Thanks a lot. It's very helpful.
Quote:Original post by rip-off
Apart from the fact that "null" isn't a valid value, yes they are all correct.
Not quite.
delete[] a[];
Also, there is almost never a reason to handle the allocations at this level. There are other, better reasons to understand pointers :)
int count = rand() % 1000;A **array = new A*[count];for(int i = 0 ; i < count ; ++i) { array = new A();}// ...for(int i = 0 ; i < count ; ++i) { delete array;}delete [] array;Should be
int count = rand() % 1000;A **array = new A*[count];for(int i = 0 ; i < count ; ++i) { array = new A();}// ...for(int i = 0 ; i < count ; ++i) { delete []array;}delete []array;
Quote:Original post by MARS_999Why's that?
*** Source Snippet Removed ***
Should be
*** Source Snippet Removed ***
Quote:Original post by MARS_999
*** Source Snippet Removed ***
Should be
*** Source Snippet Removed ***
Unless I see something wrong, no it shouldn't. You're getting an A* that points to a single element by the array access, because it was allocated as new A().
Anyway, serves as a good example why plain C pointers for array indexing in C++ is an unnecessary source of bugs.
Quote:Original post by Beyond_Repair
Anyway, serves as a good example why plain C pointers for array indexing in C++ is an unnecessary source of bugs.
But array indexing is defined with pointer arithmetic, so indexing through pointers is actually more natural than indexing with real arrays :)
x[y] is just syntactic sugar for *(x+y)
Quote:Original post by MARS_999
jyk look here
link
That thread is talking about something else. The code you pasted above is still wrong:
int count = rand() % 1000;A **array = new A*[count]; // HERE you allocate array with new[].for(int i = 0 ; i < count ; ++i) { array = new A(); // HERE, array is allocated with new.}// ...for(int i = 0 ; i < count ; ++i) { delete []array; // HERE, array is released with delete[] (WRONG)}delete []array; // HERE, array is released with delete[] (correct).What you allocate with new, you release with delete. What you allocate with new[], you release with delete[]. Mix them, and you are wrong.
I assume you were referring to MikeP's correction of your first post in that linked thread; in that thread, you are allocating with new[] in one place and freeing with delete in another. In this thread you are doing exactly the opposite. Both are incorrect.
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