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; }

因为你正在使用std::string，你正在使用c++。如果使用c++11或更高版本，则不需要任何花哨的东西。如果words是vector<string>，那么:

    for (auto & str : words) {
        for(auto & ch : str)
            ch = tolower(ch);
    }

没有奇怪的异常。可能想要使用w_char's，但除此之外，这应该全部到位。

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);
}

试试这个函数吧。

string toLowerCase(string str) {

    int str_len = str.length();

    string final_str = "";

    for(int i=0; i<str_len; i++) {

        char character = str[i];

        if(character>=65 && character<=92) {

            final_str += (character+32);

        } else {

            final_str += character;

        }

    }

    return final_str;

}

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;

看看优秀的c++17 cppp -unicodelib (GitHub)。它是单文件且只包含头文件。

#include <exception>
#include <iostream>
#include <codecvt>

// cpp-unicodelib, downloaded from GitHub
#include "unicodelib.h"
#include "unicodelib_encodings.h"

using namespace std;
using namespace unicode;

// converter that allows displaying a Unicode32 string
wstring_convert<codecvt_utf8<char32_t>, char32_t> converter;

std::u32string  in = U"Je suis là!";
cout << converter.to_bytes(in) << endl;

std::u32string  lc = to_lowercase(in);
cout << converter.to_bytes(lc) << endl;

输出

Je suis là!
je suis là!