C++ 相当于 C99 复合文字

Question

I'm new to modern C++. I'm writing a serializer and need to write a single byte (a delimiter) out. In C99 I would use a compound literal to hold the value, something like this:

enum {
   TAG_DELIMITER,
   // ...
   TAG_MAX
};

void write_buf(out_buf *dest, char *src, size_t n);

void encode(out_buf *buf, user_data *d) {
   // ... Write out user data
   write_buf(buf, &(char){TAG_DELIMITER}, sizeof(char));
}

In C++ the setup is similar:

class UserData {
public:
  // ... Some data

  void encode(std::ostream &buf) {
    // ... Write out user data
    char tmp = TAG_DELIMITER;
    buf.write(&tmp, sizeof(tmp));
  }
}

Declaring a tmp variable feels clumsy. This isn't a big deal, and I could just use extensions to enable compound literals if I really cared. But is there a "C++-ish" way of doing this I'm missing? Targeting C++20 on GCC 10.1

EDIT: To clarify, the values from the enum are used all over the code base so I'm not going to make it into a bunch of constexprs inside a class. In any case that's just moving my tmp variable into a static class member, which really isn't any better.

This question perhaps would have been better stated, "Is there a way to declare and get the address of an anonymous variable in C++?" To which I think the answer is No.

In any case, the comments were right that what I really want here is just ostream.put(TAG_DELIMITER) which neatly side steps the whole question.

Answer 1

Youn can use constexpr :

class UserData {
public:
    constexpr static char TAG_DELIMITER = 'a';
  // ... Some data

  void encode(std::ostream &buf) {
    // ... Write out user data
    const char * tmp = &TAG_DELIMITER;
    buf.write(tmp, sizeof(char));
  }
};

However, when you try to take its address, then you need to use const char * .