Re: The std::to_string/to_wstring functions should allow the user to specify the base of the numeric conversion

Ulrich Eckhardt <>
Wed, 10 Aug 2011 14:13:11 CST
Ricky65 wrote:

C++0x provides the useful std::to_string/to_wstring functions for
converting numeric values to strings. However, I was disappointed to
learn that users can't specify the base of the conversion; at the
moment the std::to_string/to_wstring functions only convert a numeric
to a base ten string. I think this is an oversight and find this very

I've been using similar functions for quite some time. In no case did I ever
find the need to specify the formatting. OTOH, I rarely use these to
stringize builtin types either, because these can already be written to a
stream as they are. That said, I do sometimes provide overloads of
to_string() for my own custom data types. These then sometimes use a
different base for numbers.

In contrast , the similar non-standard but widespread C function itoa
allows the user to specify the base of the conversion. e.g.
hexadecimal, octal and binary. For example:

int x = 23;
char buffer [32];
itoa(x, buffer, 2) //represent the value as a string in base 2

I find it rather embarrassing that a C function, albeit non-standard,
has more functionality than these C++ functions.

I tend to let personal feeling out of such discussions. I'd rather be
embarrassed writing code like the above, which overflows the buffer as soon
x becomes large. ;)

I understand that the overloaded std::to_string/to_wstring functions
call sprintf and wsprintf. The problem with this approach is that
sprintf can only convert to decimal, hexadecimal and octal bases. Why
do these functions have to use sprintf? I don't know why it couldn't
have been left to implementers to write their own versions of these
functions without these limitations.

I don't know the exact specification and I'm not sure I understand what you
are saying here. Is your only complaint that sprintf doesn't allow
formatting as binary?

It would be preferable if the user could select the base of the
numeric conversion.

The solution is simple, provide an extra argument for the base with a
default argument of 10. For example:

std::string to_string(int val, int base = 10);

There are two reasons against this:
1. creeping featuritis
The next one will want a flag to specify whether hex numbers have a 0x
prefix or not. Then someone will want to specify that the hex digits use
uppercase A-F. Then, flexible zero-padding will be requested. Don't even get
me started on other data types, those offer another set of possible flags
and options.

This would be backward compatible with the current functions as it
would default to base ten (decimal) conversion as the functions
currently do whilst at the same time allowing the user to specify the
base of the numeric conversion.

2. incompatibility
This is at the moment not that important, since the API is new, but
generally changing an API is difficult. Still, even today, a function taking
two argument is simply not one taking one argument, even if one of the two
arguments has a default. You can't use the same type of function pointer for

If something like this doesn't get implemented, I suspect people will
use itoa or its variants, or roll their own similar function, reducing
the utility of the std::to_string/to_wstring functions.

Assume a function like this:

  template<typename P1, typename P2>
  void print(char const* fmt, P1 const& p1, P2 const& p2);

This function would simply call to_string() on the given objects in order to
format them, and then replace some placeholders in 'fmt' before printing the
result. First thing to note is that there is no place here to pass
parameters, there simply is no place to put them. Still, you can use this in
order to format a number using hexadecimal:

  struct hex
     hex(unsigned v): value(v) {}
     unsigned value;

  std::string to_string(hex_value const& v)
     std::stringstream s;
     s << "0x" << std::setfill('0') << std::setwidth(8) << v.value;
     return s.str();

Note: This should determine the width from the size, make sure the value is
unsigned and be a template. Also it could avoid the overhead of a
stringstream, the size of the result is known after all.

Anyway, you can now then call:

  print("answer = %1", hex(66));

As you see, if you need you can extend this to special needs without
breaking the generality of the approach. Similarly, you can overload
to_string() for your own types.

Summary: I don't think that it's as bad as you make it sound. I do believe
that provisions like above that format numbers using different bases would
be a useful addition.


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