Const pointer C++ Stroustrup

Question

int count_x(const char* p, char x)
     // count the number of occurrences of x in p[]
     // p is assumed to point to a zero-terminated array of char (or to nothing)
{
     if (p==nullptr)
           return 0;
     int count = 0;
     for (; *p!=0; ++p)
           if (*p==x)
                 ++count;
     return count;
}

p is a pointer. The const means that the pointer cannot be modified. But in the for loop, there is ++p, which means the pointer is being iterated over/incremented to access the values *p

There’s a contradiction there - p cannot be modified yet it is being incremented/modified?

No, the `const` means the thing pointer at can't be modified. You can still change the pointer. — Galik, Sep 24 '18 at 01:17
That const means that what `p` points to is const, not `p` itself. The const would need to be on the other side of the `*` for what you describe. — Retired Ninja, Sep 24 '18 at 01:18
@Swodrfish -- yes you are correct. I had misstated the reason and removed my post as it was incorrect. Thanks! — Matt Runion, Sep 24 '18 at 01:25
I wonder why you felt it necessary to include Stroustrup's name in your question.. — user3738870, Sep 24 '18 at 01:34
Also see [What is the difference between char * const and const char *?](https://stackoverflow.com/q/890535/608639), [constant pointer vs pointer on a constant value](https://stackoverflow.com/q/10091825/608639) and friends. — jww, Sep 24 '18 at 01:35
@user3738870 The code is from *A Tour of C++* by Stroustrup. — melpomene, Sep 24 '18 at 01:38
@malpomene Thanks, so it basically denotes the author of the code in question right? — user3738870, Sep 24 '18 at 01:40

score 2 · Answer 1 · edited Sep 24 '18 at 01:33

2

Declarations in C++ are read from right to left. So something like

const char* p

would read: p is a non-const pointer to a const char.

Evidently, p is not const, but what it points to is const. So *p = 's' is illegal, but p++ is not.

edited Sep 24 '18 at 01:33

Remy Lebeau

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answered Sep 24 '18 at 01:20

DeiDei

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Wouldn't (const char)* p make more sense if that's what the line meant? Then how do you declare the pointer itself as const? – user3180 Sep 24 '18 at 01:27
@user3180 `char *const p`. – melpomene Sep 24 '18 at 01:30

Const pointer C++ Stroustrup

1 Answers1