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### 6.51 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 scope (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.