由于没有一个答案提到即将到来的Ranges库，它从c++ 20开始就在标准库中可用，目前在GitHub上单独可用为range-v3，我想添加一种使用它执行转换的方法。

就地修改字符串:

str |= action::transform([](unsigned char c){ return std::tolower(c); });

生成一个新的字符串:

auto new_string = original_string
    | view::transform([](unsigned char c){ return std::tolower(c); });

(不要忘记#include <cctype>和所需的Ranges头。)

注意:使用unsigned char作为lambda的参数是受cppreference的启发，它声明:

Like all other functions from <cctype>, the behavior of std::tolower is undefined if the argument's value is neither representable as unsigned char nor equal to EOF. To use these functions safely with plain chars (or signed chars), the argument should first be converted to unsigned char: char my_tolower(char ch) { return static_cast<char>(std::tolower(static_cast<unsigned char>(ch))); } Similarly, they should not be directly used with standard algorithms when the iterator's value type is char or signed char. Instead, convert the value to unsigned char first: std::string str_tolower(std::string s) { std::transform(s.begin(), s.end(), s.begin(), // static_cast<int(*)(int)>(std::tolower) // wrong // [](int c){ return std::tolower(c); } // wrong // [](char c){ return std::tolower(c); } // wrong [](unsigned char c){ return std::tolower(c); } // correct ); return s; }

谷歌的absl库有absl::AsciiStrToLower / absl::AsciiStrToUpper

这是Stefan Mai的回应的后续:如果你想把转换的结果放在另一个字符串中，你需要在调用std::transform之前预先分配它的存储空间。由于STL将转换后的字符存储在目标迭代器中(在每次循环迭代时递增)，因此目标字符串不会自动调整大小，并且可能会占用内存。

#include <string>
#include <algorithm>
#include <iostream>

int main (int argc, char* argv[])
{
  std::string sourceString = "Abc";
  std::string destinationString;

  // Allocate the destination space
  destinationString.resize(sourceString.size());

  // Convert the source string to lower case
  // storing the result in destination string
  std::transform(sourceString.begin(),
                 sourceString.end(),
                 destinationString.begin(),
                 ::tolower);

  // Output the result of the conversion
  std::cout << sourceString
            << " -> "
            << destinationString
            << std::endl;
}

// 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/

为ASCII字符串to_lower添加了一些可选库，它们都是生产级的，并进行了微优化，预计会比这里现有的答案更快(TODO:添加基准测试结果)。

Facebook的愚蠢:

void toLowerAscii(char* str, size_t length)

谷歌的绳降：

void AsciiStrToLower(std::string* s);

在微软平台上，您可以使用strlwr系列函数:http://msdn.microsoft.com/en-us/library/hkxwh33z.aspx

// crt_strlwr.c
// compile with: /W3
// This program uses _strlwr and _strupr to create
// uppercase and lowercase copies of a mixed-case string.
#include <string.h>
#include <stdio.h>

int main( void )
{
   char string[100] = "The String to End All Strings!";
   char * copy1 = _strdup( string ); // make two copies
   char * copy2 = _strdup( string );

   _strlwr( copy1 ); // C4996
   _strupr( copy2 ); // C4996

   printf( "Mixed: %s\n", string );
   printf( "Lower: %s\n", copy1 );
   printf( "Upper: %s\n", copy2 );

   free( copy1 );
   free( copy2 );
}