std::cout << std::fixed << std::setw(11) << std::setprecision(6) << my_double;

您需要添加

#include <iomanip>

您需要stream manipulators
你可以用任何你想要的字符来“填充”空白的地方，就像这样：

std::cout << std::fixed << std::setw(11) << std::setprecision(6) 
          << std::setfill('0') << my_double;

std::cout << boost::format("%11.6f") % my_double;

你必须要

在C++20中你可以做

double my_double = 42.0;
char str[12];
std::format_to_n(str, sizeof(str), "{:11.6}", my_double);

或

std::string s = std::format("{:11.6}", my_double);

在C++20之前的版本中，您可以使用the {fmt} library，它提供了format_to_n的实现。

• • 免责声明**：我是{fmt}和C++20 std::format的作者。

通常，您希望避免在输出点指定11和6之类的内容，这是物理标记，而您希望使用逻辑标记;例如pressure或volume。这样，您可以在一个位置定义如何格式化压力或体积，并且如果格式发生变化，您不必搜索整个程序以查找在哪里更改格式（并意外地更改其他内容的格式）在C++中，您可以通过定义一个操纵器来实现这一点，该操纵器设置各种格式选项，最好在完整表达式的末尾恢复它们。所以你最后会写这样的东西：

std::cout << pressure << my_double;

虽然我绝对不会在生产代码中使用它，但我发现下面的FFmt化程序对于快速工作很有用：

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 )
}

这允许写入如下内容：

std::cout << FFmt( 11, 6 ) << my_double;

另外，为了记录在案：

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 );
}

#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;
}

对于更喜欢使用std：：ostream的printf风格格式规范的未来访问者，这里还有另一个变体，基于Martin York在另一个SO问题中的精彩帖子：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);
    }
};

用法示例：

double my_double = 42.0;
cout << FMT("%11.6f") << my_double << "more stuff\n";

或者甚至：

int val = 42;
cout << val << " in hex is " << FMT(" 0x%x") << val << "\n";

这是我，OP，Jive Dadson -五年过去了。C++17正在成为现实。
带有完美转发功能的可变模板参数的出现让生活变得简单多了。ostream〈〈和boost：：format%的连锁疯狂可以省去。下面的函数oprintf填补了这一空白。工作正在进行中。请随意加入错误处理等内容...

#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;
}

已经有一些很好的答案了;向他们致敬！
我已经为POD类型添加了类型Assert，因为它们是printf()可用的唯一安全类型。

#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

示例用法如下。

std::cout << fmt::printf("%11.6f", my_double);

Give it a try on Coliru .