to_string or not to_string

Published Proposal,

ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++

"Though this be madness, yet there is method in ’t." ― Polonius

1. Introduction

C++11 introduced a set of std::to_string overloads for integral and floating-point types. Fortunately for integral and unfortunately for floating-point overloads they are all defined in terms of sprintf inconsistently with C++ formatted output functions ([N4910]). Additionally, the choice of the floating-point format makes std::to_string of very limited use in practice. This paper proposes fixing these issues while retaining existing semantics of integral overloads.

2. Examples

Consider the following example:

auto loc = std::locale("uk_UA.UTF-8");
setlocale(LC_ALL, "C");

std::cout << "iostreams:\n";
std::cout << 1234 << "\n";
std::cout << 1234.5 << "\n";

std::cout << "\nto_string:\n";
std::cout << std::to_string(1234) << "\n";
std::cout << std::to_string(1234.5) << "\n";

setlocale(LC_ALL, "uk_UA.UTF-8");

std::cout << "\nto_string (uk_UA.UTF-8 C locale):\n";
std::cout << std::to_string(1234) << "\n";
std::cout << std::to_string(1234.5) << "\n";

It prints:

1 234
1 234,5


to_string (uk_UA.UTF-8 C locale):

Since std::to_string uses the global C locale and no grouping the integral overloads are effectively unlocalized. The output of floating-point overloads is inconsistent with that of iostreams because the former takes the decimal point from the global C locale and doesn’t do grouping.

Additionally, due to an unfortunate choice of the fixed format in the floating-point overloads they are only useful for numbers in a limited exponent range. For example:

std::cout << std::to_string(std::numeric_limits<double>::max());


(line breaks inserted for readability)

Here only the first 17 digits are meaningful, the next 292 are so-called "garbage" digits ([DRAGON]). And finally we have 6 meaningless zeros after a possibly localized decimal point.

Formatting of small floating-point numbers is even less useful. For example:

std::cout << std::to_string(-1e-7);



3. Proposal

Redefine std::to_string in terms of std::format which in turn uses std::to_chars making more explicit the fact that integral overloads are unlocalized and changing the format of floating-point overloads to also be unlocalized and use the shortest decimal representation.

The following table shows the changes in output for the following code:

setlocale(LC_ALL, "C");
auto output = std::to_string(input);
input output
before after
42 42 42
0.42 0.420000 0.42
-1e-7 -0.000000 -1e-7
1.7976931348623157e+308 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858368.000000 1.7976931348623157e+308

and similarly with the global C locale set:

setlocale(LC_ALL, "uk_UA.UTF-8");
auto output = std::to_string(input);
input output
before after
12345 12345 12345
1234.5 1234,500000 1234.5

4. Impact on existing code

This change will affect the output of std::to_string with floating-point arguments. In most cases it will result in a more precise and/or shorter output. In cases where the C locale is explicitly set the decimal point will no longer be localized.

5. Implementation

{fmt} implements proposed changes in fmt::to_string.

6. Wording

Modify subsection "Numeric conversions [string.conversions]":

string to_string(int val);
string to_string(unsigned val);
string to_string(long val);
string to_string(unsigned long val);
string to_string(long long val);
string to_string(unsigned long long val);
string to_string(float val);
string to_string(double val);
string to_string(long double val);
7 Returns: Each function returns a string object holding the character representation of the value of its argument that would be generated by calling sprintf(buf, fmt, val) with a format specifier of "%d", "%u", "%ld", "%lu", "%lld", "%llu", "%f", "%f", or "%Lf", respectively, where buf designates an internal character buffer of sufficient size.

7 Returns: format("{}", val).


Informative References

Guy L. Steele Jr.; Jon L White. How to Print Floating-Point Numbers Accurately. URL: https://fmt.dev/papers/p372-steele.pdf
Thomas Köppe. Working Draft, Standard for Programming Language C++. URL: https://wg21.link/n4910