Compiler incompatibility

[Anthony Appleyard]

I wrote a Visual C++ program which in an .h file had the lines

char*charnames[256]={"00",
"lbutton",
"rbutton",
"cancel",
"mbutton",
....etc....

That always compiled OK under my old Visual C++ compiler. But the new compiler rejects it with "a value of type "const char*" cannot be used to initialize an entity of type "char*".

That fault also happens with the lines
#define PROGNAME "makeobj"
#define TEXPROGNAME "makeobj texturemap"

char*progname = PROGNAME, *texprogname = TEXPROGNAME;

I had other faults with objects of the mode char* used in an = assignment or as a function call parameter, which my old compiler accepted and my new compiler rejects. Please, is there a list of these changes in what the compiler accepts and what it rejects? Is there a workaround for these changes?

[salem c]

Yes, make them all const char *
"strings" were made constants in C about 30 years ago, so there's really no excuse at all for being tardy.

[JLBorges]

> is there a list of these changes?

Annex C of the standard 'Compatibity' lists the major changes across versions.
http://eel.is/c++draft/diff


> Is there a workaround for these changes?

For this specific change, C.6.2/4 http://eel.is/c++draft/diff.lex#4 has

Change: String literals made const.
The type of a string-literal is changed from “array of char” to “array of const char”. ...

Effect on original feature: Change to semantics of well-defined feature.

Difficulty of converting: Syntactic transformation. The fix is to add a cast:

char* p = "abc";                // valid in C, invalid in C++
void f(char*) {
  char* p = (char*)"abc";       // OK: cast added
  f(p);
  f((char*)"def");              // OK: cast added
}

[Anthony Appleyard]

Thanks.

In this new version, how can I tell the compiler to treat all 'char*' declarations as 'long char*'?

[seeplus]

?? long char* ??

long char* isn't a valid type. Long means long integer and char* is a pointer to memory that contains data of type char.

Do you mean a cast from char* to long?

How are you using this? Can you give an example code?

Note that C++ specification has been tightened over the years and some 'flaky' code that might have compiled and run Ok years ago no probably won't.

[Anthony Appleyard]

The offending statement is

GetDlgItemText(db, 11, C, 1024);


where GetDlgItemText is automatically defined as GetDlgItemTextW

and C is a local variable char C[1024];

[seeplus]

The compiler settings are set to use wide-char as default (hence the W). If you aren't using wide-chars in your program, the easiest way is to change the compiler settings.

Project/properties/Advanced/Character Set. Change to Use Multi-Byte Character Set

Alternatively, you can specifiy that the ASCII versions of the API's are used by specifiying an A at the end of the name (instead of the W)

Note that there are probably other settings that should/need to be changed - ie general/c++ language standard. It defaults to c++14. set to either C++20 or preview if you want to also use some features from C++23.