some questions about const

The const keyword can be placed at different places.

Code:

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const int *ptr3 = 0;
int const *ptr4 = 0;

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Code:

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const int a = 5; //can not be changed
int const b = 6;//can also not be changed

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What`s the difference?

In this case, your preference is the only difference.

If you would want a const pointer to int instead of a pointer to const int, placement does matter.

The only time it matters is with pointers.
If the const comes before the * as in: const char* string; then that means the string cannot be changed, but the pointer can be changed.
If the const comes after the * as in: char* const string; then that means the pointer to string cannot be changed, but the string itself can be changed.