6.48 Function Names as Strings

GCC provides three magic constants that hold the name of the current function as a string. In C++11 and later modes, all three are treated as constant expressions and can be used in constexpr constexts. The first of these constants is __func__, which is part of the C99 standard:

The identifier __func__ is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration

     static const char __func__[] = "function-name";

appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function. As an extension, at file (or, in C++, namespace scope), __func__ evaluates to the empty string.

__FUNCTION__ is another name for __func__, provided for backward compatibility with old versions of GCC.

In C, __PRETTY_FUNCTION__ is yet another name for __func__, except that at file (or, in C++, namespace scope), it evaluates to the string "top level". In addition, in C++, __PRETTY_FUNCTION__ contains the signature of the function as well as its bare name. For example, this program:

     extern "C" int printf (const char *, ...);
     
     class a {
      public:
       void sub (int i)
         {
           printf ("__FUNCTION__ = %s\n", __FUNCTION__);
           printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
         }
     };
     
     int
     main (void)
     {
       a ax;
       ax.sub (0);
       return 0;
     }

gives this output:

     __FUNCTION__ = sub
     __PRETTY_FUNCTION__ = void a::sub(int)

These identifiers are variables, not preprocessor macros, and may not be used to initialize char arrays or be concatenated with string literals.