How do I do the following with std::cout?
double my_double = 42.0;
char str[12];
printf_s("%11.6lf", my_double); // Prints " 42.000000"
I am just about ready to give up and use sprintf_s.
More generally, where can I find a reference on std::ostream formatting that lists everything in one place, rather than spreading it all out in a long tutorial?
EDIT Dec 21, 2017 - See my answer below. It uses features that were not available when I asked this question in 2012.
In C++20 you'll be able to do
double my_double = 42.0;
char str[12];
std::format_to_n(str, sizeof(str), "{:11.6}", my_double);
or
std::string s = std::format("{:11.6}", my_double);
In the meantime you can use the {fmt} library that provides an implementation of format_to_n.
Disclaimer: I'm the author of {fmt} and C++20 std::format.
For future visitors who prefer actual printf-style format specs with std::ostream, here is yet another variation, based on Martin York's excellent post in another SO question: https://stackoverflow.com/a/535636:
#include <iostream>
#include <iomanip>
#include <stdio.h> //snprintf
class FMT
{
public:
explicit FMT(const char* fmt): m_fmt(fmt) {}
private:
class fmter //actual worker class
{
public:
explicit fmter(std::ostream& strm, const FMT& fmt): m_strm(strm), m_fmt(fmt.m_fmt) {}
//output next object (any type) to stream:
template<typename TYPE>
std::ostream& operator<<(const TYPE& value)
{
// return m_strm << "FMT(" << m_fmt << "," << value << ")";
char buf[40]; //enlarge as needed
snprintf(buf, sizeof(buf), m_fmt, value);
return m_strm << buf;
}
private:
std::ostream& m_strm;
const char* m_fmt;
};
const char* m_fmt; //save fmt string for inner class
//kludge: return derived stream to allow operator overloading:
friend FMT::fmter operator<<(std::ostream& strm, const FMT& fmt)
{
return FMT::fmter(strm, fmt);
}
};
usage example:
double my_double = 42.0;
cout << FMT("%11.6f") << my_double << "more stuff\n";
or even:
int val = 42;
cout << val << " in hex is " << FMT(" 0x%x") << val << "\n";
Some great answers already; kudos to those!
This is based on some of them. I have added type assertions for POD types, since they are the only safe types usable with printf().
#include <iostream>
#include <stdio.h>
#include <type_traits>
namespace fmt {
namespace detail {
template<typename T>
struct printf_impl
{
const char* fmt;
const T v;
printf_impl(const char* fmt, const T& v) : fmt(fmt), v(v) {}
};
template<typename T>
inline typename std::enable_if<std::is_pod<T>::value, std::ostream& >::type
operator<<(std::ostream& os, const printf_impl<T>& p)
{
char buf[40];
::snprintf(buf, sizeof(buf), p.fmt, p.v, 40);
return os << buf;
}
} // namespace detail
template<typename T>
inline typename std::enable_if<std::is_pod<T>::value, detail::printf_impl<T> >::type
printf(const char* fmt, const T& v)
{
return detail::printf_impl<T>(fmt, v);
}
} // namespace fmt
Example usage it as below.
std::cout << fmt::printf("%11.6f", my_double);