conversion between char * and const char *

Question

Fl_Widget have next constructor:

Fl_Widget (int x, int y, int w, int h, const char *label=0L)

How can i send char* instead const char*? I just want use my char Tmp[255] variable in constructor.

Just sent it. A `const char*` parameter can receive a `char*`. — Galik, Oct 23 '14 at 10:43
Did you try just sending it? `char*` (and hence `char[255]`) is convertible to `const char*`. — Mike Seymour, Oct 23 '14 at 10:43
@deviantfan Or `NULL` or `0`, if you don't have C++11. The `0L` is confusing. — James Kanze, Oct 23 '14 at 11:23

Bathsheba · Answer 1 · 2014-10-23T10:55:32.620

3

C++ allows the implicit casting of a T* to a const T* for any type T so in your case you can just pass the parameter to the function.

Also, if you own the Fl_Widget constructor, then you should change the default value for label from 0L to nullptr, or 0 if using C++03 or earlier.

(To convert from const T* to T* requires a const_cast and can lead to undefined behaviour).

edited Oct 23 '14 at 10:55

answered Oct 23 '14 at 10:49

Bathsheba

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In more general terms `const` can always be implicitly added, but not removed without `const_cast`, which should basically never be used. – bcrist Oct 23 '14 at 11:58

score 0 · Answer 2 · answered Oct 23 '14 at 10:59

If your ctor currently looks like this, where foo is your class:

foo::foo() : FL_Widget(13, 13, 13, 13, Tmp){
    char Tmp[255]{"blah blah blah"};
}

That's not legal. You cannot pass a parameter to a parent ctor that happens in your class's ctor, this is because the parent ctor happens before your class ctor is called.

You could instead do this: foo::foo() : FL_Widget(13, 13, 13, 13, "blah blah blah"){}

conversion between char * and const char *

2 Answers2