C++ mark as deprecated

Question

In C++14, you can mark a function as deprecated using the [[deprecated]] attribute (see section 7.6.5 [dcl.attr.deprecated]).

The attribute-token deprecated can be used to mark names and entities whose use is still allowed, but is discouraged for some reason.

For example, the following function foo is deprecated:

[[deprecated]]
void foo(int);

It is possible to provide a message that describes why the name or entity was deprecated:

[[deprecated("Replaced by bar, which has an improved interface")]]
void foo(int);

The message must be a string literal.

For further details, see “Marking as deprecated in C++14”.

This should do the trick:

#ifdef __GNUC__
#define DEPRECATED(func) func __attribute__ ((deprecated))
#elif defined(_MSC_VER)
#define DEPRECATED(func) __declspec(deprecated) func
#else
#pragma message("WARNING: You need to implement DEPRECATED for this compiler")
#define DEPRECATED(func) func
#endif

...

//don't use me any more
DEPRECATED(void OldFunc(int a, float b));

//use me instead
void NewFunc(int a, double b);

However, you will encounter problems if a function return type has a commas in its name e.g. std::pair<int, int> as this will be interpreted by the preprocesor as passing 2 arguments to the DEPRECATED macro. In that case you would have to typedef the return type.

Edit: simpler (but possibly less widely compatible) version here.

Here's a simplified version of my 2008 answer:

#if defined(__GNUC__) || defined(__clang__)
#define DEPRECATED __attribute__((deprecated))
#elif defined(_MSC_VER)
#define DEPRECATED __declspec(deprecated)
#else
#pragma message("WARNING: You need to implement DEPRECATED for this compiler")
#define DEPRECATED
#endif

//...

//don't use me any more
DEPRECATED void OldFunc(int a, float b);

//use me instead
void NewFunc(int a, double b);

See also:

• MSVC documentation for __declspec(deprecated)
• GCC documentation for __attribute__((deprecated))
• Clang documentation for __attribute__((deprecated))

In GCC you can declare your function with the attribute deprecated like this:

void myfunc() __attribute__ ((deprecated));

This will trigger a compile-time warning when that function is used in a .c file.

You can find more info under "Diagnostic pragmas" at http://gcc.gnu.org/onlinedocs/gcc/Pragmas.html

Here is a more complete answer for 2018.

These days, a lot of tools allow you to not just mark something as deprecated, but also provide a message. This allows you to tell people when something was deprecated, and maybe point them toward a replacement.

There is still a lot of variety in compiler support:

• C++14 supports [[deprecated]]/[[deprecated(message)]].
• __attribute__((deprecated)) is supported by GCC 4.0+ and ARM 4.1+
• __attribute__((deprecated)) and __attribute__((deprecated(message))) is supported for:
  • GCC 4.5+
  • Several compilers which masquerade as GCC 4.5+ (by setting __GNUC__/__GNUC_MINOR__/__GNUC_PATCHLEVEL__)
  • Intel C/C++ Compiler going back to at least 16 (you can't trust __GNUC__/__GNUC_MINOR__, they just set it to whatever version of GCC is installed)
  • ARM 5.6+
• MSVC supports __declspec(deprecated) since 13.10 (Visual Studio 2003)
• MSVC supports __declspec(deprecated(message)) since 14.0 (Visual Studio 2005)

You can also use [[gnu::deprecated]] in recent versions of clang in C++11, based on __has_cpp_attribute(gnu::deprecated).

I have some macros in Hedley to handle all of this automatically which I keep up to date, but the current version (v2) looks like this:

#if defined(__cplusplus) && (__cplusplus >= 201402L)
#  define HEDLEY_DEPRECATED(since) [[deprecated("Since " #since)]]
#  define HEDLEY_DEPRECATED_FOR(since, replacement) [[deprecated("Since " #since "; use " #replacement)]]
#elif \
  HEDLEY_GCC_HAS_EXTENSION(attribute_deprecated_with_message,4,5,0) || \
  HEDLEY_INTEL_VERSION_CHECK(16,0,0) || \
  HEDLEY_ARM_VERSION_CHECK(5,6,0)
#  define HEDLEY_DEPRECATED(since) __attribute__((__deprecated__("Since " #since)))
#  define HEDLEY_DEPRECATED_FOR(since, replacement) __attribute__((__deprecated__("Since " #since "; use " #replacement)))
#elif \
  HEDLEY_GCC_HAS_ATTRIBUTE(deprcated,4,0,0) || \
  HEDLEY_ARM_VERSION_CHECK(4,1,0)
#  define HEDLEY_DEPRECATED(since) __attribute__((__deprecated__))
#  define HEDLEY_DEPRECATED_FOR(since, replacement) __attribute__((__deprecated__))
#elif HEDLEY_MSVC_VERSION_CHECK(14,0,0)
#  define HEDLEY_DEPRECATED(since) __declspec(deprecated("Since " # since))
#  define HEDLEY_DEPRECATED_FOR(since, replacement) __declspec(deprecated("Since " #since "; use " #replacement))
#elif HEDLEY_MSVC_VERSION_CHECK(13,10,0)
#  define HEDLEY_DEPRECATED(since) _declspec(deprecated)
#  define HEDLEY_DEPRECATED_FOR(since, replacement) __declspec(deprecated)
#else
#  define HEDLEY_DEPRECATED(since)
#  define HEDLEY_DEPRECATED_FOR(since, replacement)
#endif

I'll leave it as an exercise to figure out how to get rid of the *_VERSION_CHECK and *_HAS_ATTRIBUTE macros if you don't want to use Hedley (I wrote Hedley largely so I wouldn't have to think about that on a regular basis).

If you use GLib, you can use the G_DEPRECATED and G_DEPRECATED_FOR macros. They're not as robust as the ones from Hedley, but if you already use GLib there is nothing to add.