问题
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.
回答1:
std::cout << std::fixed << std::setw( 11 ) << std::setprecision( 6 ) << my_double;
You need to add
#include <iomanip>
You need stream manipulators
You may "fill" the empty places with whatever char you want. Like this:
std::cout << std::fixed << std::setw( 11 ) << std::setprecision( 6 )
<< std::setfill( '0' ) << my_double;
回答2:
std::cout << boost::format("%11.6f") % my_double;
You have to #include <boost\format.hpp>
回答3:
#include <iostream>
#include <iomanip>
int main() {
double my_double = 42.0;
std::cout << std::fixed << std::setw(11)
<< std::setprecision(6) << my_double << std::endl;
return 0;
}
回答4:
In general, you want to avoid specifying things like 11 and 6 at the
point of output. That's physical markup, and you want logical markup;
e.g. pressure, or volume. That way, you define in a single place
how pressure or volume are formatted, and if that formatting changes,
you don't have to search through out the program to find where to change
the format (and accidentally change the format of something else). In
C++, you do this by defining a manipulator, which sets the various
formatting options, and preferrably restores them at the end of the full
expression. So you end up writing things like:
std::cout << pressure << my_double;
Although I definitly wouldn't use it in production code, I've found the
following FFmt formatter useful for quicky jobs:
class FFmt : public StateSavingManip
{
public:
explicit FFmt(
int width,
int prec = 6,
std::ios::fmtflags additionalFlags
= static_cast<std::ios::fmtflags>(),
char fill = ' ' );
protected:
virtual void setState( std::ios& targetStream ) const;
private:
int myWidth;
int myPrec;
std::ios::fmtflags myFlags;
char myFill;
};
FFmt::FFmt(
int width,
int prec,
std::ios::fmtflags additionalFlags,
char fill )
: myWidth( width )
, myPrec( prec )
, myFlags( additionalFlags )
, myFill( fill )
{
myFlags &= ~ std::ios::floatfield
myFlags |= std::ios::fixed
if ( isdigit( static_cast< unsigned char >( fill ) )
&& (myFlags & std::ios::adjustfield) == 0 ) {
myFlags |= std::ios::internal
}
}
void
FFmt::setState(
std::ios& targetStream ) const
{
targetStream.flags( myFlags )
targetStream.width( myWidth )
targetStream.precision( myPrec )
targetStream.fill( myFill )
}
This allows writing things like:
std::cout << FFmt( 11, 6 ) << my_double;
And for the record:
class StateSavingManip
{
public:
StateSavingManip(
StateSavingManip const& other );
virtual ~StateSavingManip();
void operator()( std::ios& stream ) const;
protected:
StateSavingManip();
private:
virtual void setState( std::ios& stream ) const = 0;
private:
StateSavingManip& operator=( StateSavingManip const& );
private:
mutable std::ios* myStream;
mutable std::ios::fmtflags
mySavedFlags;
mutable int mySavedPrec;
mutable char mySavedFill;
};
inline std::ostream&
operator<<(
std::ostream& out,
StateSavingManip const&
manip )
{
manip( out );
return out;
}
inline std::istream&
operator>>(
std::istream& in,
StateSavingManip const&
manip )
{
manip( in );
return in;
}
StateSavingManip.cc:
namespace {
// We maintain the value returned by ios::xalloc() + 1, and not
// the value itself. The actual value may be zero, and we need
// to be able to distinguish it from the 0 resulting from 0
// initialization. The function getXAlloc() returns this value
// -1, so we add one in the initialization.
int getXAlloc();
int ourXAlloc = getXAlloc() + 1;
int
getXAlloc()
{
if ( ourXAlloc == 0 ) {
ourXAlloc = std::ios::xalloc() + 1;
assert( ourXAlloc != 0 );
}
return ourXAlloc - 1;
}
}
StateSavingManip::StateSavingManip()
: myStream( NULL )
{
}
StateSavingManip::StateSavingManip(
StateSavingManip const&
other )
{
assert( other.myStream == NULL );
}
StateSavingManip::~StateSavingManip()
{
if ( myStream != NULL ) {
myStream->flags( mySavedFlags );
myStream->precision( mySavedPrec );
myStream->fill( mySavedFill );
myStream->pword( getXAlloc() ) = NULL;
}
}
void
StateSavingManip::operator()(
std::ios& stream ) const
{
void*& backptr = stream.pword( getXAlloc() );
if ( backptr == NULL ) {
backptr = const_cast< StateSavingManip* >( this );
myStream = &stream;
mySavedFlags = stream.flags();
mySavedPrec = stream.precision();
mySavedFill = stream.fill();
}
setState( stream );
}
回答5:
it's me, the OP, Jive Dadson - five years on. C++17 is becoming a reality.
The advent of variadic template parameters with perfect forwarding has made life so much simpler. The chained madness of ostream<< and boost::format% can be dispensed with. The function oprintf below fills the bill. Work in progress. Feel free to chime in on error-handling, etc...
#include <iostream>
#include <string.h>
#include <stdio.h>
#include <string_view>
namespace dj {
template<class Out, class... Args>
Out& oprintf(Out &out, const std::string_view &fmt, Args&&... args) {
const int sz = 512;
char buffer[sz];
int cx = snprintf(buffer, sz, fmt.data(), std::forward<Args>(args)...);
if (cx >= 0 && cx < sz) {
return out.write(buffer, cx);
} else if (cx > 0) {
// Big output
std::string buff2;
buff2.resize(cx + 1);
snprintf(buff2.data(), cx, fmt.data(), std::forward<Args>(args)...);
return out.write(buff2.data(), cx);
} else {
// Throw?
return out;
}
}
}
int main() {
const double my_double = 42.0;
dj::oprintf(std::cout, "%s %11.6lf\n", "My double ", my_double);
return 0;
}
回答6:
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";
来源：https://stackoverflow.com/questions/11989374/floating-point-format-for-stdostream