字段宽度

设置字段宽度非常简单。对于每个变量，只需在其前面加上“setw（n）"。如下所示：

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
  const int max = 12;
  const int width = 6;
  for(int row = 1; row <= max; row++) {
      for(int col = 1; col <= max; col++) {
          cout << setw(width) << row * col;
      }
      cout << endl;
  }
  return 0;
}

注意“setw（n）”是如何控制字段宽度的，因此每个数字都打印在一个字段内，该字段保持相同的宽度，而不管数字本身的宽度如何。
--从"Programming/C++ tutorial" by P. Lutus开始。

样本输出：

2017-12-20T16:24:47,604144+01:00 Hello, World!

代码（put_printf用法在put_timestamp中演示）：

#include <assert.h>
#include <chrono>
#include <iomanip>
#include <iostream>

class put_printf {
    static constexpr size_t failed = std::numeric_limits<size_t>::max(); // for any explicit error handling
    size_t stream_size; // excluding '\0'; on error set to 0 or to "failed"
    char buf_stack[2048+1]; // MAY be any size that fits on the stack (even 0), SHOULD be (just) large enough for most uses (including '\0')
    std::unique_ptr<char[]> buf_heap; // only used if the output doesn't fit in buf_stack
public:
    explicit put_printf(const char *format, ...)
            #if __GNUC__
            __attribute__ ((format (printf, 2, 3))) // most compelling reason for not using a variadic template; parameter 1 is implied "this"
            #endif
            {
        va_list args;
        va_start(args, format);
        const int res = vsnprintf(buf_stack, sizeof(buf_stack), format, args);
        va_end(args);
        if (res < 0) { // easily provoked, e.g., with "%02147483646i\n", i.e., more than INT_MAX-1 significant characters (only observed, no guarantee seen)
            stream_size = failed;
        } else if (res < sizeof(buf_stack)) { // preferred path
            stream_size = res;
        } else { // not artificially constrained
            try {
                const size_t buf_size = static_cast<size_t>(res) + 1; // avoids relying on "res < INT_MAX" (only observed, no guarantee seen)
                buf_heap.reset(new char[buf_size]); // observed to work even beyond INT_MAX=2^32-1 bytes
                va_start(args, format);
                if (vsnprintf(buf_heap.get(), buf_size, format, args) == res) stream_size = res;
                else stream_size = failed; // can't happen
                va_end(args);
            } catch (const std::bad_alloc&) { // insufficient free heap space (or an environment-specific constraint?)
                stream_size = failed;
            }
        }
    }
    friend std::ostream& operator<<(std::ostream& os, const put_printf& self) {
        if (self.stream_size == failed) {
            // (placeholder for any explicit error handling)
            return os;
        } else {
            // using write() rather than operator<<() to avoid a separate scan for '\0' or unintentional truncation at any internal '\0' character
            return os.write((self.buf_heap ? self.buf_heap.get() : self.buf_stack), self.stream_size);
        }
    }
};

class put_timestamp {
    const bool basic = false;
    const bool local = true;
public:
    friend std::ostream& operator<<(std::ostream& os, const put_timestamp& self) {
        const auto now = std::chrono::system_clock::now();
        const std::time_t now_time_t = std::chrono::system_clock::to_time_t(now);
        struct tm tm; if ((self.local ? localtime_r(&now_time_t, &tm) : gmtime_r(&now_time_t, &tm)) == nullptr) return os; // TODO: explicit error handling?
        static_assert(4 <= sizeof(int), "");
        const int microseconds = std::chrono::duration_cast<std::chrono::microseconds>(now.time_since_epoch() % std::chrono::seconds(1)).count();
        assert(0 <= microseconds && microseconds < 1000000); // TODO: (how) do we know?
        // TODO: doesn't "point" in "decimal_point()" imply "dot"/"full stop"/"period", unlike an obviously neutral term like "mark"/"separator"/"sign"?
        const char decimal_sign = std::use_facet<std::numpunct<char>>(os.getloc()).decimal_point() == '.' ? '.' : ','; // full stop accepted, comma preferred
        // TODO: all well and good for a locale-specific decimal sign, but couldn't the locale also upset microseconds formatting by grouping digits?
        os << std::put_time(&tm, self.basic ? "%Y%m%dT%H%M%S" : "%FT%T") << put_printf("%c%06i", decimal_sign, microseconds);
        if (! self.local) return os << "Z";
        const int tz_minutes = std::abs(static_cast<int>(tm.tm_gmtoff)) / 60;
        return os << put_printf(self.basic ? "%c%02i%02i" : "%c%02i:%02i", 0 <= tm.tm_gmtoff ? '+' : '-', tz_minutes / 60, tz_minutes % 60);
    }
};

int main() {
    // testing decimal sign
    ///std::cout.imbue(std::locale("en_GB"));
    ///std::cout.imbue(std::locale("fr_FR"));

    std::cout << put_timestamp() << " Hello, World!\n";
    #if 0
    typedef put_printf pf; // just to demo local abbreviation
    std::cout << "1: " << pf("%02147483646i\n"  , 1     ) << std::endl; // res < 0
    std::cout << "2: " << pf("%02147483643i%i\n", 1, 100) << std::endl; // res < 0
    std::cout << "3: " << pf("%02147483643i%i\n", 1,  10) << std::endl; // works
    std::cout << "4: " << pf("%02147483646i"    , 1     ) << std::endl; // works
    #endif
    return 0;
}

关于put_printf的注解：

// Reasons for the name "put_printf" (and not "putf" after all):
// - put_printf is self-documenting, while using the naming pattern also seen in std::put_time;
// - it is not clear whether the proposed std::putf would support exactly the same format syntax;
// - it has a niche purpose, so a longer name is not an objection, and for frequent local uses
//     it is easy enough to declare an even shorter "typedef put_printf pf;" or so.
// Evaluation of delegating to vsnprintf() with intermediate buffer:
// (+) identical result without implementation and/or maintenance issues,
// (?) succeeds or fails as a whole, no output of successful prefix before point of failure
// (-) (total output size limited to INT_MAX-1)
// (-) overhead (TODO: optimal buf_stack size considering cache and VM page locality?)
// Error handling (an STL design problem?):
// - std::cout.setstate(std::ios_base::failbit) discards further std::cout output (stdout still works),
//     so, to be aware of an error in business logic yet keep on trucking in diagnostics,
//     should there be separate classes, or a possibility to plug in an error handler, or what?
// - should the basic or default error handling print a diagnostic message? throw an exception?
// TODO: could a function "int ostream_printf(std::ostream& os, const char *format, ...)"
//           first try to write directly into os.rdbuf() before using buf_stack and buf_heap,
//           and would that significantly improve performance or not?

当我同时需要cout的类型安全性和printf（）简单变量的快速和简单格式时，我就像这样混合使用这两种方法，这是一个丑陋的修复，但是当我需要输出像“02/07/2014 10：05 am”这样的内容以及一些更复杂的实体时，它可以为我完成任务：

#include <stdio>
#include <stdarg>
#include <stdlib>
#include <iostream>

#pragma hdrstop

using namespace std;

char* print(char* fmt, ...)
{
    static char buffer[80] = "";

    va_list argptr;
    va_start(argptr,fmt);

    vsprintf(buffer, fmt, argptr);

    va_end(argptr);

    return buffer;
}

#pragma argsused
int main(int argc, char* argv[])
{

cout << print("\n%06d\n%6d\n%6d\n%010.3f",1,12,123,123.456);

system("PAUSE>NUL");

return 0;

}

在C++20中，你可以使用std::format来实现安全的printf：

std::cout << std::format("The answer is {}.\n", 42);

除此之外，std::format所基于的{fmt} library还提供了print函数，该函数将格式化和输出结合在一起：

fmt::print("The answer is {}.\n", 42);

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

您可以直接使用std::ostream执行的唯一操作是众所周知的<<-语法：

int i = 0;
std::cout << "this is a number: " << i;

有各种各样的IO manipulators可以用来影响整数、浮点数等的格式、位数等。
然而，这与printf的格式化字符串不同。C11不包含任何允许您以与printf相同的方式使用字符串格式化的工具（除了printf本身，当然如果您愿意，可以在C中使用它）。
就提供printf风格功能的库而言，有boost::format，它支持如下代码（从概要中复制）：

std::cout << boost::format("writing %1%,  x=%2% : %3%-th try") % "toto" % 40.23 % 50;

还要注意的是，在标准的未来版本中有printf样式格式的proposal for inclusion。如果这被接受，可能会出现如下语法：

std::cout << std::putf("this is a number: %d\n",i);

这是我已经习惯的一个成语，希望它能帮上忙：

// Hacky but idiomatic printf style syntax with c++ <<

#include <cstdlib> // for sprintf

char buf[1024]; sprintf(buf, "%d score and %d years ago", 4, 7);
cout << string(buf) <<endl;

&

我建议使用ostringstream而不是ostream，请参见以下示例：

#include <vector>
#include <string>
#include <iostream>
#include "CppUnitTest.h"

#define _CRT_NO_VA_START_VALIDATION

std::string format(const std::string& format, ...)
{
    va_list args;
    va_start(args, format);
    size_t len = std::vsnprintf(NULL, 0, format.c_str(), args);
    va_end(args);
    std::vector<char> vec(len + 1);
    va_start(args, format);
    std::vsnprintf(&vec[0], len + 1, format.c_str(), args);
    va_end(args);
    return &vec[0];
}

示例用法：

std::ostringstream ss;
ss << format("%s => %d", "Version", Version) << std::endl;
Logger::WriteMessage(ss.str().c_str()); // write to unit test output
std::cout << ss.str() << std::endl; // write to standard output

要实现printf，可以使用c++11模板参数：

#include <iostream>
#include <string>

inline std::ostream & mprintf(std::ostream & ostr, const char * fstr) throw()
{
    return ostr << fstr;
}

template<typename T, typename... Args> 
std::ostream & mprintf(std::ostream & ostr, 
        const char * fstr, const T & x) throw()
{
    size_t i=0;
    char c = fstr[0];

    while (c != '%')
    {
        if(c == 0) return ostr; // string is finished
        ostr << c;
        c = fstr[++i];
    };
    c = fstr[++i];
    ostr << x;

    if(c==0) return ostr; // 

    // print the rest of the stirng
    ostr << &fstr[++i];
    return ostr;
}

template<typename T, typename... Args> 
std::ostream & mprintf(std::ostream & ostr,
        const char * fstr, const T & x, Args... args) throw()
{
    size_t i=0;
    char c = fstr[0];

    while (c != '%')
    {
        if(c == 0) return ostr; // string is finished
        ostr << c;
        c = fstr[++i];
    };
    c = fstr[++i];
    ostr << x;

    if(c==0) return ostr; // string is finished

    return mprintf(ostr, &fstr[++i], args...);
}

int main()
{
    int c = 50*6;
    double a = 34./67.;
    std::string q = "Hello!";

    // put only two arguments
    // the symbol after % does not matter at all
    mprintf(std::cout, "%f + %f = %a \n", c, a);

    // print string object: for real printf one should write q.c_str() 
    mprintf(std::cout, "message: \"%s\". \n", q);

    // the last argument will be ignored
    mprintf(std::cout, "%z + %f\n", (long)a, 12, 544 );

}

• • 产出**

300 + 2 = %a 
message: "Hello!". 
2 + 12

这是一个非常简单的代码，它可以改进.
1)优点是它使用〈〈将对象打印到流中，因此您可以放置任意参数，这些参数可以通过〈〈输出。
2)它忽略格式化字符串中参数的类型：%后面可以是任意符号，甚至是空格。2输出流决定如何打印相应的对象。3它与printf兼容。
3)一个缺点是它不能打印百分比符号' %'，需要稍微改进代码。
4)它不能打印格式化的数字，如%4.5f
5)如果参数的数目少于格式化字符串所预测的数目，则函数只打印字符串的其余部分。
6)如果参数的数目大于格式化字符串预测的数目，则忽略剩余的参数
你可以改进代码使2）-6）完全模仿printf的行为，但是，如果你遵循printf的规则，那么只有3）和4）需要被修正。

我独立编写，但得出的答案与user3283405相似
我的解决方案使用vasprintf（）来实现格式设置，并使用std：：ostream的〈〈操作符重载来释放正确位置的内存。
用法：

std::cout << putf(const char *format, ...); //Same format as C printf(3)

代码：

#define _GNU_SOURCE
#include <cstdarg>
#include <iostream>
#include <cstdio>

struct putf_r{
        char *s;
};

putf_r putf(const char *fmt, ...){
        va_list ap;
        va_start(ap, fmt);
        putf_r a;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
        vasprintf(&a.s, fmt, ap);
#pragma GCC diagnostic pop
        va_end(ap);
        return a;
}

std::ostream& operator<<(std::ostream& os, putf_r a){
        os<<a.s;
        free(a.s);
        return os;
}

int main(){
        std::cout << putf("%3d\n", 23) << putf("%a\n", 256.);
}

请注意，编译器不会检查putf（）中的格式，因此编译器标志-Wformat-nonliteral不会对putf（）中的可疑代码发出警告，您需要自己关注uncontrolled format string问题。
详细信息可在GitHub上找到