如何使用具有类似 printf 格式的 C++ std::ostream？

Question

我正在学习 C++。 cout 是 std::ostream 类的一个实例。 如何用它打印格式化的字符串？

我仍然可以使用printf，但我想学习一种可以利用所有 C++ 优势的正确 C++ 方法。我认为std::ostream 应该可以做到这一点，但我找不到正确的方法。

Answer 1

您可以直接使用std::ostream 做的唯一事情是众所周知的<<-syntax：

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

还有各种IO manipulators可以用来影响整数、浮点数等的格式、位数等。

但是，这与printf 的格式化字符串不同。 C++11 不包括任何允许您以与printf 相同的方式使用字符串格式的工具（printf 本身除外，如果您愿意，当然可以在 C++ 中使用它）。

就提供printf-style 功能的库而言，有boost::format，它启用了这样的代码（从概要复制）：

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

另请注意，在标准的未来版本中存在printf 样式的proposal for inclusion 格式。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);

Answer 2

在 C++20 中，您将能够使用 std::format 进行安全的 printf 类格式设置：

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

除了{fmt} library，std::format基于，提供了print结合格式化和输出的功能：

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

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

Answer 3

这是我习惯的成语。希望对您有所帮助：

// 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;

&

Answer 4

我建议使用 ostringstream 而不是 ostream 请参阅以下示例：

#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

Answer 5

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 );

}

输出

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

这是一个非常简单的代码，可以改进。

1) 优点是它使用 输出的任意参数

2) 它忽略格式化字符串中参数的类型：在 % 之后可以代表任意符号，甚至是空格。输出流决定如何打印相应的对象。它还兼容 printf。

3) 缺点是不能打印百分号'%'，需要稍微改进一下代码。

4) 无法打印格式化数字，如 %4.5f

5) 如果参数的数量少于格式化字符串所预测的数量，则该函数只打印字符串的其余部分。

6) 如果参数的数量大于格式化字符串预测的数量，则忽略剩余的参数

可以改进代码以使 2)-6) 完全模仿 printf 行为。 但是，如果你遵循 printf 的规则，那么基本上只需要修复 3) 和 4)。

Answer 6

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)”。像这样：

#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，所以每个数字都是 打印在保持相同宽度的字段内，无论 数字本身的宽度。

-- 来自"Programming/C++ tutorial" by P. Lutus。

Answer 7

我自己写的，但想出了类似于 user3283405 的答案

我的解决方案使用 vasprintf() 来实现格式化，并使用 std::ostream 的

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，您需要自己处理 uncontrolled format string 问题。
详细信息可见GitHub

Answer 8

样本输出：

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

代码（在 put_timestamp 中演示了 put_printf 的用法）：

#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 的评论：

// 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?

Answer 9

当我需要 cout 的类型安全性和 printf() 简单变量的快速简便格式化时，我会像这样混合两者。这是一个丑陋的修复，但是当我需要输出诸如“02/07/2014 10:05am”之类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;

}