Is It Legal to Redefine a C++ Keyword

Is it legal to redefine a C++ keyword?

17.4.3.1.1 Macro names [lib.macro.names]

1 Each name defined as a macro in a header is reserved to the implementation for any use if the translation unit includes the header.164)

2 A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

By the way, new is an operator and it can be overloaded (replaced) by the user by providing its own version.

C++ preprocessor #define-ing a keyword. Is it standards conforming?

In C++, the closest thing to forbidding #defineing a keyword is §17.4.3.1.1/2, which only disallows it in a translation unit that includes a standard library header:

A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

The second sentence of that paragraph has been changed in C++0x to outright forbid #defineing a keyword (C++0x FCD §17.6.3.3.1):

A translation unit shall not #define or #undef names lexically identical to keywords.

Edit: As pointed out by Ken Bloom in comments to his answer, the rules have not changed in C++0x; the text has just been rearranged to confuse people like me. :-)

Can I redefine a C++ macro then define it back?

As greyfade pointed out, your ___T___ trick doesn't work because the preprocessor is a pretty simple creature. An alternative approach is to use pragma directives:

 // juice includes here
 #pragma push_macro("T")
 #undef T
 // include boost headers here
 #pragma pop_macro("T")

That should work in MSVC++ and GCC has added support for pop_macro and push_macro for compatibility with it. Technically it is implementation-dependent though, but I don't think there's a standard way of temporarily suppressing the definition.

Redefinition allowed in C but not in C++?

Tentative definition is allowed in C but not in C++.

A tentative definition is any external data declaration that has no storage class specifier and no initializer.

C99 6.9.2/2

A declaration of an identifier for an object that has file scope without an initializer, and
without a storage-class specifier or with the storage-class specifier static, constitutes a
tentative definition. If a translation unit contains one or more tentative definitions for an
identifier, and the translation unit contains no external definition for that identifier, then
the behavior is exactly as if the translation unit contains a file scope declaration of that
identifier, with the composite type as of the end of the translation unit, with an initializer
equal to 0.

So int i is a tentative definition. The C compiler will combine all of the tentative definitions into a single definition of i.

In C++ your code is ill-formed due to the One Definition Rule (Section 3.2/1 ISO C++)

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template.

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// but if I write int i = 5; again I get error in C also

Because in that case it no longer remains a tentative definition because of the initializer (5).

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Just for the sake of information

J.5.11 Multiple external definitions

There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).

Also check out this excellent post on external variables.

Can I mimic a C header that redefines bool in C++?

You can hack it!

The library, call it fooLib, thinks it's using some type bool which it has the prerogative to define. To the library, bool is just an identifier.

So, you can just force it to use another identifier instead:

#define bool fooLib_bool
#include "fooLib.h"
#undef bool
#undef true
#undef false

Now the compiler sees the offending line transformed to this:

typedef int fooLib_bool;

You're stuck with the interface using type fooLib_bool = int instead of a real bool, but that's impossible to work around, as the code might in fact rely on the properties of int, and library binary would have been compiled with such an assumption baked in.

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