Why no std::string overload for std::to_string

Question

The title says it all really. Why did they choose to not have a

namespace std
{
    std::string to_string(const std::string&)
}

overload?

I have an program that can query some data by row index or row name; ideal for templates. Until I try to construct an error message if a row being accessed is missing:

template <typename T>
int read(T nameOrIndex)
{
    if (!present(nameOrIndex))
    {
        // This does not compile if T is std::string.
        throw std::invalid_argument("Missing row: " + std::to_string(nameOrIndex));
    }
}

I could add my own overload to the std namespace but that's not ideal.

Your code will not compile if T is any number of other types too. What about all of those? Should there be a to_string implementation that returns some string for arbitrary types? Instead of adding to_string overloads to std (which is illegal), I'd create my own to_string overloads in the same namespace as read. Delegate to std::to_string from that, and return the argument as is from the overload that takes std::string const&. — Praetorian
– Praetorian, Commented Apr 12, 2016 at 22:43
std::to_string converts built-in numeric types to strings. There's no corresponding need to "convert" strings to strings because they already are strings. — Pete Becker
– Pete Becker, Commented Apr 12, 2016 at 23:35
As I understand, to_string is basically a safe interface to the C formatting utilities that conceals the buffer and all that from you. There is a C formatting specifier %s for strings. Thus, something has clearly been lost :) — Chris Beck
– Chris Beck, Commented Jun 21, 2016 at 11:28
@PeteBecker: It would add consistency and potentially ease some code. Despite a lack of need, you can also convert an integer to an integer, e.g. int x; int y = static_cast<int>(x);. — Sebastian Mach
– Sebastian Mach, Commented Oct 16, 2018 at 8:40

Timothy Shields · Accepted Answer · 2016-04-12 23:42:26Z

13

Rather than define your own overload taking std::string in the std namespace, which is a very bad solution, just do this instead:

#include <iostream>
#include <string>

std::string to_string(const std::string& value)
{
    return value;
}

template <typename T>
void display(T value)
{
    using namespace std;
    std::cout << to_string(value) << std::endl;
}

int main()
{
    display("ABC");
    display(7);
}

edited Apr 12, 2016 at 23:42

answered Apr 12, 2016 at 23:09

Timothy Shields

80.3k21 gold badges127 silver badges173 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

M.M Over a year ago

This code doesn't use ADL, because the arguments have builtin type. Instead, the display function brings in names from std, so that unqualified lookup has both std::to_string and ::to_string in the lookup set.

Marc.2377 Over a year ago

@M.M, what does ADL mean in this context?

Mic Over a year ago

I remember that "using namespace" is not a good practice for embedded software, according to AUTOSAR. I don't have the exact rule though.

5gon12eder · Accepted Answer · 2016-04-12 23:12:32Z

2

I'm not going to guess why the overload is not in the standard library but here is a simple solution you can use. Instead of std::to_string, use boost::lexical_cast.

template <typename T>
int read(T nameOrIndex)
{
  if (!present(nameOrIndex))
    {
      throw std::invalid_argument {
          "Missing row: " + boost::lexical_cast<std::string>(nameOrIndex)
      };
    }
  // …
}

If you don't want the Boost dependency, you can easily roll your own function for this.

template <typename T>
std::string
to_string(const T& obj)
{
  std::ostringstream oss {};
  oss << obj;
  return oss.str();
}

It is obviously not optimized but might be good enough for your case.

answered Apr 12, 2016 at 23:12

5gon12eder

25.5k6 gold badges51 silver badges99 bronze badges

1 Comment

Mic Over a year ago

ostringstream is definitely the most cross-type solution. It fixes the weird fact that std::to_string doesn't work for std::string arg

Collectives™ on Stack Overflow

Why no std::string overload for std::to_string

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related