cout.fill( '0' );    
cout.width( 3 );
cout << value;

我会使用下面的函数。我不喜欢sprintf;这不是我想要的!!

#define hexchar(x)    ((((x)&0x0F)>9)?((x)+'A'-10):((x)+'0'))
typedef signed long long   Int64;

// Special printf for numbers only
// See formatting information below.
//
//    Print the number "n" in the given "base"
//    using exactly "numDigits".
//    Print +/- if signed flag "isSigned" is TRUE.
//    Use the character specified in "padchar" to pad extra characters.
//
//    Examples:
//    sprintfNum(pszBuffer, 6, 10, 6,  TRUE, ' ',   1234);  -->  " +1234"
//    sprintfNum(pszBuffer, 6, 10, 6, FALSE, '0',   1234);  -->  "001234"
//    sprintfNum(pszBuffer, 6, 16, 6, FALSE, '.', 0x5AA5);  -->  "..5AA5"
void sprintfNum(char *pszBuffer, int size, char base, char numDigits, char isSigned, char padchar, Int64 n)
{
    char *ptr = pszBuffer;

    if (!pszBuffer)
    {
        return;
    }

    char *p, buf[32];
    unsigned long long x;
    unsigned char count;

    // Prepare negative number
    if (isSigned && (n < 0))
    {
        x = -n;
    }
    else
    {
        x = n;
    }

    // Set up small string buffer
    count = (numDigits-1) - (isSigned?1:0);
    p = buf + sizeof (buf);
    *--p = '\0';

    // Force calculation of first digit
    // (to prevent zero from not printing at all!!!)
    *--p = (char)hexchar(x%base);
    x = x / base;

    // Calculate remaining digits
    while(count--)
    {
        if(x != 0)
        {
            // Calculate next digit
            *--p = (char)hexchar(x%base);
            x /= base;
        }
        else
        {
            // No more digits left, pad out to desired length
            *--p = padchar;
        }
    }

    // Apply signed notation if requested
    if (isSigned)
    {
        if (n < 0)
        {
            *--p = '-';
        }
        else if (n > 0)
        {
            *--p = '+';
        }
        else
        {
            *--p = ' ';
        }
    }

    // Print the string right-justified
    count = numDigits;
    while (count--)
    {
        *ptr++ = *p++;
    }
    return;
}

cout.fill( '0' );    
cout.width( 3 );
cout << value;

在单个数字值的实例上使用零作为填充字符输出日期和时间的另一个示例:2017-06-04 18:13:02

#include "stdafx.h"
#include <iostream>
#include <iomanip>
#include <ctime>
using namespace std;

int main()
{
    time_t t = time(0);   // Get time now
    struct tm * now = localtime(&t);
    cout.fill('0');
    cout << (now->tm_year + 1900) << '-'
        << setw(2) << (now->tm_mon + 1) << '-'
        << setw(2) << now->tm_mday << ' '
        << setw(2) << now->tm_hour << ':'
        << setw(2) << now->tm_min << ':'
        << setw(2) << now->tm_sec
        << endl;
    return 0;
}

cout.fill('*');
cout << -12345 << endl; // print default value with no field width
cout << setw(10) << -12345 << endl; // print default with field width
cout << setw(10) << left << -12345 << endl; // print left justified
cout << setw(10) << right << -12345 << endl; // print right justified
cout << setw(10) << internal << -12345 << endl; // print internally justified

这将产生输出:

-12345
****-12345
-12345****
****-12345
-****12345

下面，

#include <iomanip>
#include <iostream>

int main()
{
    std::cout << std::setfill('0') << std::setw(5) << 25;
}

输出将是

00025

Setfill默认设置为空格字符(' ')。Setw设置要打印的字段的宽度，仅此而已。

如果你有兴趣了解如何格式化输出流，我写了另一个问题的答案，希望它是有用的: 格式化c++控制台输出。