Boost为此提供了一个字符串算法:

#include <boost/algorithm/string.hpp>

std::string str = "HELLO, WORLD!";
boost::algorithm::to_lower(str); // modifies str

或者，对于非原位:

#include <boost/algorithm/string.hpp>

const std::string str = "HELLO, WORLD!";
const std::string lower_str = boost::algorithm::to_lower_copy(str);

如果字符串包含ASCII范围之外的UTF-8字符，则boost::algorithm::to_lower将不会转换这些字符。当涉及UTF-8时，最好使用boost::locale::to_lower。参见http://www.boost.org/doc/libs/1_51_0/libs/locale/doc/html/conversions.html

使用此代码在c++中更改字符串的大小写。

#include<bits/stdc++.h>

using namespace std;

int main(){
  string a = "sssAAAAAAaaaaDas";
  transform(a.begin(),a.end(),a.begin(),::tolower);
  cout<<a;
}

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);
}

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; }