Re: C++11 Variadic Templates: Format("I %%% about %%% of %%%","dream", 7, 9) == "I dream about 7 of 9"

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 29 Nov 2011 22:33:56 -0800 (PST)
On 2011-11-28 23:31, Andrew Tomazos wrote:

I wanted to write a function that takes a string containing a number
of placeholders and a corresponding number of other parameters
(perhaps non-pod) and returns a string with the placeholders replaced
with stringified versions of the parameters:

Yes, the idea is nice, but let me comment on some parts.

I haven't used Variadic Templates before. My first draft is below. It
appears to work, but it's using a linear recursion. My question is,
is there a way to write it iteratively?

Yes, see below.


using namespace std;

class StringFormatException {};

const string g_sPlaceholder("%%%");

template<class ... T>
inline string Format(const string& sFormat, const T& ...);

This function template is no-where use. You should get rid of it.

template<class ... T>
inline string Format(const string& sFormat)

There is no reason to declare this as a variadic template. Just declare
it as a non-template function:

inline string Format(const string& sFormat)

         size_t iFirstPlaceholderPos = sFormat.find(g_sPlaceholder);

Either use string::size_type or - more preferably auto to determine the
type of iFirstPlaceholderPos. In theory there is no guarantee that
size_t and string::size_type have the same type or even the same maximum
value (which is relevant when using string::npos).

         if (iFirstPlaceholderPos != string::npos)
                 throw StringFormatException();

         return sFormat;

template<class Head, class ... Tail>
inline string Format(const string& sFormat, const Head& head, const
Tail& ... tail)
         stringstream os;

         size_t iFirstPlaceholderPos = sFormat.find(g_sPlaceholder);

Same problem here.

         if (iFirstPlaceholderPos == string::npos)
                 throw StringFormatException();
                 string sFront(sFormat, 0, iFirstPlaceholderPos);
                 string sBack(sFormat, iFirstPlaceholderPos +

                 os<< sFront<< head<< Format(sBack, tail ... );

         return os.str();

int main()
         if (Format("I %%% about %%% of %%%","dream", 7, 9) == "I dream
about 7 of 9")
                 cout<< "pass"<< endl;
                 cout<< "fail"<< endl;

         return 0;

In regard to your question, whether an iterative solution is possible:
Yes, at least in the sense that you can serialize your expansions. The
following code demonstrates this:

#include <string>
#include <sstream>
#include <iostream>
#include <cstddef>

class StringFormatException {};

const std::string g_sPlaceholder("%%%");

struct void_{};

template<class T>
void_ DoFormat(std::ostream& os, const std::string& sFormat,
              std::string::size_type& curr, const T& t)
  auto nextPos = sFormat.find(g_sPlaceholder, curr);
  if (nextPos == std::string::npos)
    throw StringFormatException();
    os.write(&sFormat.front() + curr, nextPos - curr);
    os << t;
    curr = nextPos + g_sPlaceholder.size();
  return {};

template <class... Args>
inline std::string Format(const std::string& sFormat, const Args&... args)
  std::stringstream os;
  std::string::size_type curr = 0;
  std::initializer_list<void_>{DoFormat(os, sFormat, curr, args)...};
  auto nextPos = sFormat.find(g_sPlaceholder, curr);
  if (nextPos != std::string::npos)
    throw StringFormatException();
  if (!sFormat.empty())
    os.write(&sFormat.front() + curr, sFormat.size() - curr);
  return os.str();

// ... Your test main() follows here unchanged

You may wonder about the astonishing void_ type as well as the
std::initializer_list<void_>{DoFormat(os, sFormat, curr, args)...}

What I'm realizing here is to create an initializer-list expanded with
sizeof...(Args) expressions that have type void_. Unfortunately I cannot
use void here, because this is no object type. Even more unfortunate is,
that I cannot use a function call e.g.

template<class... T>
void swallow(T&&...){}


swallow(DoFormat(os, sFormat, curr, args)...);

This will compile, but since the order of the evaluation of function
arguments is not determined, there is no guarantee that this works in a
portable manner, because each function call modifies the arguments os
and curr. There exists a special rule in C++11 that ensures that
initializer-lists are evaluated in a strict order, this is the reason
why I'm creating an otherwise unused std::initializer_list<void_> here.

As elegant as it looks, I'm very unhappy about the need to create such a
hack here. It would be great, if there would exist a further expansion
pattern that allows me to write e.g.

DoFormat(os, sFormat, curr, args)...;

such that DoFormat may have type void as well.

I hope nonetheless that this suggestion gives you another idea how to

HTH & Greetings from Bremen,

Daniel Kr?gler

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