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About multi level pointer

t im (45)
Here is a code about multi level pointer:
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double* (*a)[3][6];
cout << sizeof(a) << endl;  // 4
cout << sizeof(*a) << endl;  // 72
cout << sizeof(**a) << endl;  // 24
cout << sizeof(***a) << endl;  // 4
cout << sizeof(****a) << endl;  // 8

I am confused about the output of the code. Can someone help me analyse the meaning of every expression in sizeof and how can I analyse it?
Thanks for your help.
Peter87 (11235)
Think about what type each expression is.

 
 
cout << sizeof(a) << endl;  // 4
a is a pointer to an array of 3 arrays of 6 pointers to double. On your system/with your compiler a pointer seems to be 4 bytes.

 
 
cout << sizeof(*a) << endl;  // 72
*a is an array of 3 arrays of 6 pointers to double. Since the size of a pointer is 4 bytes and you have 3×6 pointers the total size is 3×6×4=72 bytes. 

 
 
cout << sizeof(**a) << endl;  // 24
**a is an array of 6 pointers to double. 6×4=24 bytes.

 
 
cout << sizeof(***a) << endl;  // 4
***a is a pointer to double. As we have said before, the size of a pointer for you is 4 bytes.

 
 
cout << sizeof(****a) << endl;  // 8
****a is a double which is 8 bytes.

If we replace the expressions with types instead your code will look like this:
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cout << sizeof(double*(*)[3][6]) << endl;  // 4
cout << sizeof(double*[3][6]) << endl;  // 72
cout << sizeof(double*[6]) << endl;  // 24
cout << sizeof(double*) << endl;  // 4
cout << sizeof(double) << endl;  // 8
This code is equivalent to what you had and will give the same outputs.
Last edited on
t im (45)
Thanks a lot for your detailed reply. I got it.
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