Re: [boost] Variadic append for std::string

Hi, Am 23.01.2017 19:26, schrieb Olaf van der Spek:

On Mon, Jan 23, 2017 at 5:32 PM, Christof Donat wrote:

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1. scope for formatting tags:

concat(format::hex<int>, 42, " is hex for ", concat(42)).str();

Here the inner concat will convert the 42 to its decimal representation, while the outer one converts the first 42 to its hex representation.

Wouldn't concat(hex(42), " is hex for", 42) make more sense?

That is a valid approach for concat() and format(), but suboptimal for join(). Think of this example: join(hex<int>, separator(" "), my_nums); Here all the numbers are converted to their hex representation. With your approach this would look like: join(separator(" "), my_nums | transform([](int i) -> int { return hex(i); })); That is much more difficult to understand. Since for join() tag parameters to define the conversion is, to me, the superior choice, I think, we should use it for concat() and format() as well, for consistency.

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2. concat() in calls to format():

format("%|1$40t|%2%", concat(first_name, " ", last_name), phone_number).str();

Why not fold the name concat into the format string?

In this example I want the full name to take up 40 characters, no matter if the first name is long or short. With format strings as used by boost::format I don't know how that could be achieved, and extending the format language, of course, makes the interpreter slower and more complex.

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If "concat" is the outer layer anyway, I would return a std::string directly for convenience. It is easy to forget the trailing .str() and it does not look elegant.

Of course better proposals are welcome :-) Would you prefer the implicit conversion? If so, why?

Implicit is problematic with auto..

That is one of the reasons, I prefer the explicit str() function. It also fits well to other factory functions, we might prefer to have as well, like e.g. concat(...).append_to(my_string), or concat(...).overwrite(my_string). Christof