CODEWITHSUNDEEP
c++
sooqua
sooqua

Reputation: 438

C++ Why const LPSTR is different than const char *?

Why does the following code compiles?

void foo(const LPSTR str) {
    str[0] = '\0';
}

while

void foo(LPCSTR str) {
    str[0] = '\0';
}

and

void foo(const char* str) {
    str[0] = '\0';
}

does not.

It's actually LPTSTR in my code so const version is LPCTSTR... So can I increase code readability by having something like const LPTSTR, or it must be either LPCTSTR or const TCHAR*?

Upvotes: 3

Views: 3118

Answers (4)

eerorika
eerorika

Reputation: 238351

If you take a look at the documentation you'll find that LPSTR is an alias of CHAR *, while CHAR is in turn an alias of char. Therefore LPSTR is an alias of char*.

Why const LPSTR is different than const char *?

const in const char * applies to the char. In other words, it is a non-const pointer to const object.

const in const LPSTR applies to LPSTR, which as we have discovered is char*. Because LPSTR is a pointer, this makes the pointer const. It does not have an effect on whether the pointer points to a const or non-const object. So, the type is char * const.

As for why does str[0] = '\0'; not compile when str is const char*, the reason is that you may not modify const objects. It compiles with char * const, because you may modify non-const objects.

PS. The fact that this question exists, demonstrates how hiding the data-pointer types behind aliases is confusing.

Upvotes: 2

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

Because a LPCSTR is not the same as a (hypothetical) CLPSTR.

Upvotes: -3

Pumkko
Pumkko

Reputation: 1593

  1. const LPSTR is resolved as char* const : Only the pointer is constant you can modify its content. Look at its definition here. So your first example compile
  2. LPCSTRis resolved as const char* so you can't modify its content. You can see its definition here. So your second example does not compile
  3. Again with const char* you can not modify its content, so your last example does not compile

Upvotes: 0

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

A typedef "seals" the type from outside modification. LPSTR is a char *, period. Adding const to that (as const LPSTR) adds the const to the outside: you get a char * const.

What you want is to "inject" the const (apply it the pointee, not to the pointer), and that is not possible through a simple declaration syntax with a typedef. So it must be LPCSTR, the typedef created for exactly this purpose.

Upvotes: 8

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