我刚刚测试了这个，它工作得很好，不需要查找表。Rand_alnum()在某种程度上强制输出字母数字，但因为它从可能的256个字符中选择62个，所以这不是什么大问题。

#include <cstdlib>   // for rand()
#include <cctype>    // for isalnum()   
#include <algorithm> // for back_inserter
#include <string>

char 
rand_alnum()
{
    char c;
    while (!std::isalnum(c = static_cast<char>(std::rand())))
        ;
    return c;
}


std::string 
rand_alnum_str (std::string::size_type sz)
{
    std::string s;
    s.reserve  (sz);
    generate_n (std::back_inserter(s), sz, rand_alnum);
    return s;
}

下面是我用c++ 11改编的Ates Goral的答案。我在这里添加了lambda，但原理是你可以传入它，从而控制你的字符串包含什么字符:

std::string random_string( size_t length )
{
    auto randchar = []() -> char
    {
        const char charset[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";
        const size_t max_index = (sizeof(charset) - 1);
        return charset[ rand() % max_index ];
    };
    std::string str(length,0);
    std::generate_n( str.begin(), length, randchar );
    return str;
}

下面是一个传递lambda到随机字符串函数的例子:http://ideone.com/Ya8EKf

为什么要用c++ 11呢?

因为您可以为感兴趣的字符集生成遵循特定概率分布(或分布组合)的字符串。 因为它内置了对非确定性随机数的支持 因为它支持unicode，所以你可以把它改成国际化的版本。

例如:

#include <iostream>
#include <vector>
#include <random>
#include <functional> //for std::function
#include <algorithm>  //for std::generate_n

typedef std::vector<char> char_array;

char_array charset()
{
    //Change this to suit
    return char_array( 
    {'0','1','2','3','4',
    '5','6','7','8','9',
    'A','B','C','D','E','F',
    'G','H','I','J','K',
    'L','M','N','O','P',
    'Q','R','S','T','U',
    'V','W','X','Y','Z',
    'a','b','c','d','e','f',
    'g','h','i','j','k',
    'l','m','n','o','p',
    'q','r','s','t','u',
    'v','w','x','y','z'
    });
};    

// given a function that generates a random character,
// return a string of the requested length
std::string random_string( size_t length, std::function<char(void)> rand_char )
{
    std::string str(length,0);
    std::generate_n( str.begin(), length, rand_char );
    return str;
}

int main()
{
    //0) create the character set.
    //   yes, you can use an array here, 
    //   but a function is cleaner and more flexible
    const auto ch_set = charset();

    //1) create a non-deterministic random number generator      
    std::default_random_engine rng(std::random_device{}());

    //2) create a random number "shaper" that will give
    //   us uniformly distributed indices into the character set
    std::uniform_int_distribution<> dist(0, ch_set.size()-1);

    //3) create a function that ties them together, to get:
    //   a non-deterministic uniform distribution from the 
    //   character set of your choice.
    auto randchar = [ ch_set,&dist,&rng ](){return ch_set[ dist(rng) ];};

    //4) set the length of the string you want and profit!        
    auto length = 5;
    std::cout<<random_string(length,randchar)<<std::endl;
    return 0;
}

样例输出。

 void gen_random(char *s, size_t len) {
     for (size_t i = 0; i < len; ++i) {
         int randomChar = rand()%(26+26+10);
         if (randomChar < 26)
             s[i] = 'a' + randomChar;
         else if (randomChar < 26+26)
             s[i] = 'A' + randomChar - 26;
         else
             s[i] = '0' + randomChar - 26 - 26;
     }
     s[len] = 0;
 }

//C++ Simple Code
#include <bits/stdc++.h>
using namespace std;
int main() {
vector<char> alphanum =
    {'0','1','2','3','4',
'5','6','7','8','9',
'A','B','C','D','E','F',
'G','H','I','J','K',
'L','M','N','O','P',
'Q','R','S','T','U',
'V','W','X','Y','Z',
'a','b','c','d','e','f',
'g','h','i','j','k',
'l','m','n','o','p',
'q','r','s','t','u',
'v','w','x','y','z'
};
string s="";
int len=5;
srand(time(0)); 
for (int i = 0; i <len; i++) {
    int t=alphanum.size()-1;
    int idx=rand()%t;
    s+= alphanum[idx];
}
cout<<s<<" ";
return 0;
}

我刚刚测试了这个，它工作得很好，不需要查找表。Rand_alnum()在某种程度上强制输出字母数字，但因为它从可能的256个字符中选择62个，所以这不是什么大问题。

#include <cstdlib>   // for rand()
#include <cctype>    // for isalnum()   
#include <algorithm> // for back_inserter
#include <string>

char 
rand_alnum()
{
    char c;
    while (!std::isalnum(c = static_cast<char>(std::rand())))
        ;
    return c;
}


std::string 
rand_alnum_str (std::string::size_type sz)
{
    std::string s;
    s.reserve  (sz);
    generate_n (std::back_inserter(s), sz, rand_alnum);
    return s;
}

标准库中最合适的函数是std::sample:

#include <algorithm>
#include <iterator>
#include <random>
#include <string>
#include <iostream>

static const char charset[] = 
    "0123456789"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "abcdefghijklmnopqrstuvwxyz";

template<typename URBG>
std::string gen_string(std::size_t length, URBG&& g) {
    std::string result;
    result.resize(length);
    std::sample(std::cbegin(charset), 
                std::cend(charset),
                std::begin(result),
                std::intptr_t(length),
                std::forward<URBG>(g));
    return result;
}

int main() {
    std::mt19937 g;
    std::cout << gen_string(10, g) << std::endl;
    std::cout << gen_string(10, g) << std::endl;
}

随机数生成器的状态应该在调用之间保持在函数之外。