另一种方法是使用带参考变量的基于范围的for循环

string test = "Hello World";
for(auto& c : test)
{
   c = tolower(c);
}

cout<<test<<endl;

我写了这个简单的helper函数:

#include <locale> // tolower

string to_lower(string s) {        
    for(char &c : s)
        c = tolower(c);
    return s;
}

用法:

string s = "TEST";
cout << to_lower("HELLO WORLD"); // output: "hello word"
cout << to_lower(s); // won't change the original variable.

我写了一个模板版本，适用于任何字符串:

#include <type_traits> // std::decay
#include <ctype.h>    // std::toupper & std::tolower


template <class T = void> struct farg_t { using type = T; };
template <template<typename ...> class T1, 
class T2> struct farg_t <T1<T2>> { using type = T2*; };
//---------------

template<class T, class T2 = 
typename std::decay< typename farg_t<T>::type >::type>
void ToUpper(T& str) { T2 t = &str[0]; 
for (; *t; ++t) *t = std::toupper(*t); }


template<class T, class T2 = typename std::decay< typename 
farg_t<T>::type >::type>
void Tolower(T& str) { T2 t = &str[0]; 
for (; *t; ++t) *t = std::tolower(*t); }

用gcc编译器测试:

#include <iostream>
#include "upove_code.h"

int main()
{

    std::string str1 = "hEllo ";
    char str2 [] = "wOrld";

    ToUpper(str1);
    ToUpper(str2);
    std::cout << str1 << str2 << '\n'; 
    Tolower(str1);
    Tolower(str2);
    std::cout << str1 << str2 << '\n'; 
    return 0;
}

输出:

>HELLO WORLD
>
>hello world

// tolower example (C++)
#include <iostream>       // std::cout
#include <string>         // std::string
#include <locale>         // std::locale, std::tolower

int main ()
{
  std::locale loc;
  std::string str="Test String.\n";
  for (std::string::size_type i=0; i<str.length(); ++i)
    std::cout << std::tolower(str[i],loc);
  return 0;
}

欲了解更多信息:http://www.cplusplus.com/reference/locale/tolower/

如果你想要一些简单的东西，这里有一个宏观的技巧:

#define STRTOLOWER(x) std::transform (x.begin(), x.end(), x.begin(), ::tolower)
#define STRTOUPPER(x) std::transform (x.begin(), x.end(), x.begin(), ::toupper)
#define STRTOUCFIRST(x) std::transform (x.begin(), x.begin()+1, x.begin(),  ::toupper); std::transform (x.begin()+1, x.end(),   x.begin()+1,::tolower)

但是，请注意，如果您正在处理的不仅仅是ASCII字符，那么@AndreasSpindler对这个答案的评论仍然是一个重要的考虑因素。

使用c++ 11中基于范围的for循环，代码会更简单:

#include <iostream>       // std::cout
#include <string>         // std::string
#include <locale>         // std::locale, std::tolower

int main ()
{
  std::locale loc;
  std::string str="Test String.\n";

 for(auto elem : str)
    std::cout << std::tolower(elem,loc);
}