Named parameter string formatting in C++

Question

I\'m wondering if there is a library like Boost Format, but which supports named parameters rather than positional ones. This is a common idiom in e.g. Python, where you have a

Answer 1

The fmt library supports named arguments:

print("You clicked {button} at {x},{y}.",
      arg("button", "b1"), arg("x", 50), arg("y", 30));

And as a syntactic sugar you can even (ab)use user-defined literals to pass arguments:

print("You clicked {button} at {x},{y}.",
      "button"_a="b1", "x"_a=50, "y"_a=30);

For brevity the namespace fmt is omitted in the above examples.

Disclaimer: I'm the author of this library.

Answer 2

The answer appears to be, no, there is not a C++ library that does this, and C++ programmers apparently do not even see the need for one, based on the comments I have received. I will have to write my own yet again.

Answer 3

I've writen a library for this puporse, check it out on GitHub.

Contributions are wellcome.

Answer 4

Well I'll add my own answer as well, not that I know (or have coded) such a library, but to answer to the "keep the memory allocation down" bit.

As always I can envision some kind of speed / memory trade-off.

On the one hand, you can parse "Just In Time":

class Formater:
  def __init__(self, format): self._string = format

  def compute(self):
    for k,v in context:
      while self.__contains(k):
        left, variable, right = self.__extract(k)
        self._string = left + self.__replace(variable, v) + right

This way you don't keep a "parsed" structure at hand, and hopefully most of the time you'll just insert the new data in place (unlike Python, C++ strings are not immutable).

However it's far from being efficient...

On the other hand, you can build a fully constructed tree representing the parsed format. You will have several classes like: Constant, String, Integer, Real, etc... and probably some subclasses / decorators as well for the formatting itself.

I think however than the most efficient approach would be to have some kind of a mix of the two.

• explode the format string into a list of Constant, Variable
• index the variables in another structure (a hash table with open-addressing would do nicely, or something akin to Loki::AssocVector).

There you are: you're done with only 2 dynamically allocated arrays (basically). If you want to allow a same key to be repeated multiple times, simply use a std::vector<size_t> as a value of the index: good implementations should not allocate any memory dynamically for small sized vectors (VC++ 2010 doesn't for less than 16 bytes worth of data).

When evaluating the context itself, look up the instances. You then parse the formatter "just in time", check it agaisnt the current type of the value with which to replace it, and process the format.

Pros and cons: - Just In Time: you scan the string again and again - One Parse: requires a lot of dedicated classes, possibly many allocations, but the format is validated on input. Like Boost it may be reused. - Mix: more efficient, especially if you don't replace some values (allow some kind of "null" value), but delaying the parsing of the format delays the reporting of errors.

Personally I would go for the One Parse scheme, trying to keep the allocations down using boost::variant and the Strategy Pattern as much I could.

Answer 5

Given that Python it's self is written in C and that formatting is such a commonly used feature, you might be able (ignoring copy write issues) to rip the relevant code from the python interpreter and port it to use STL maps rather than Pythons native dicts.

Answer 6

I've always been critic with C++ I/O (especially formatting) because in my opinion is a step backward in respect to C. Formats needs to be dynamic, and makes perfect sense for example to load them from an external resource as a file or a parameter.

I've never tried before however to actually implement an alternative and your question made me making an attempt investing some weekend hours on this idea.

Sure the problem was more complex than I thought (for example just the integer formatting routine is 200+ lines), but I think that this approach (dynamic format strings) is more usable.

You can download my experiment from this link (it's just a .h file) and a test program from this link (test is probably not the correct term, I used it just to see if I was able to compile).

The following is an example

#include "format.h"
#include <iostream>

using format::FormatString;
using format::FormatDict;

int main()
{
    std::cout << FormatString("The answer is %{x}") % FormatDict()("x", 42);
    return 0;
}

It is different from boost.format approach because uses named parameters and because the format string and format dictionary are meant to be built separately (and for example passed around). Also I think that formatting options should be part of the string (like printf) and not in the code.

FormatDict uses a trick for keeping the syntax reasonable:

FormatDict fd;
fd("x", 12)
  ("y", 3.141592654)
  ("z", "A string");

FormatString is instead just parsed from a const std::string& (I decided to preparse format strings but a slower but probably acceptable approach would be just passing the string and reparsing it each time).

The formatting can be extended for user defined types by specializing a conversion function template; for example

struct P2d
{
    int x, y;
    P2d(int x, int y)
        : x(x), y(y)
    {
    }
};

namespace format {
    template<>
    std::string toString<P2d>(const P2d& p, const std::string& parms)
    {
        return FormatString("P2d(%{x}; %{y})") % FormatDict()
            ("x", p.x)
            ("y", p.y);
    }
}

after that a P2d instance can be simply placed in a formatting dictionary.

Also it's possible to pass parameters to a formatting function by placing them between % and {.

For now I only implemented an integer formatting specialization that supports

1. Fixed size with left/right/center alignment
2. Custom filling char
3. Generic base (2-36), lower or uppercase
4. Digit separator (with both custom char and count)
5. Overflow char
6. Sign display

I've also added some shortcuts for common cases, for example

"%08x{hexdata}"

is an hex number with 8 digits padded with '0's.

"%026/2,8:{bindata}"

is a 24-bit binary number (as required by "/2") with digit separator ":" every 8 bits (as required by ",8:").

Note that the code is just an idea, and for example for now I just prevented copies when probably it's reasonable to allow storing both format strings and dictionaries (for dictionaries it's however important to give the ability to avoid copying an object just because it needs to be added to a FormatDict, and while IMO this is possible it's also something that raises non-trivial problems about lifetimes).

UPDATE

I've made a few changes to the initial approach:

1. Format strings can now be copied
2. Formatting for custom types is done using template classes instead of functions (this allows partial specialization)
3. I've added a formatter for sequences (two iterators). Syntax is still crude.

I've created a github project for it, with boost licensing.