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大小写转换。

std::transform

new

新创建一个std::string

#include <string>
#include <cctype>
#include <iostream>
#include <algorithm> // std::transform
std::string make_lowercase(const std::string &in)
{
    std::string out;
    std::transform(in.begin(), in.end(), std::back_inserter(out), ::tolower);
    return out;
}

int main()
{
    std::string s = "hello";
    std::string s1 = "HELLO";
    if (make_lowercase(s1) == s)
    {
        std::cout << "equal" << std::endl;
    }
    else
    {
        std::cout << "not equal" << std::endl;
    }
}
// g++ --std=c++11 test.cpp

inplace

就地修改，不新创建

#include <string>
#include <cctype>
#include <iostream>
#include <algorithm> // std::transform
void to_lowercase(std::string &in)
{
    std::transform(in.begin(), in.end(), in.begin(), ::tolower);
}

int main()
{
    std::string s = "hello";
    std::string s1 = "HELLO";
    to_lowercase(s1);
    if (s1 == s)
    {
        std::cout << "equal" << std::endl;
    }
    else
    {
        std::cout << "not equal" << std::endl;
    }
}
// g++ --std=c++11 test.cpp

static void ToUpper(std::string& S)
{
    std::transform(S.begin(), S.end(), S.begin(), ::toupper);
}

stackoverflow What does the “::” mean in “::tolower”?

NOTE: 这是"Use a name in global namespace"，参见 C++\Language-reference\Basic-concept\Organization\Scope 章节。

A

As to why the :: is necessary: the standard defines two tolower's, a function template in std::, and a simple function in both :: and std::. Depending on which headers have been included (and that includes headers indirectly included from other headers, which you may not know about), either one, the other, or both may be visible. Using :: ensures that the older one, from the C standard, is used. (If the one in std:: is considered, the call will be ambiguous, since transform is a template itself, and the compiler will not be able to deduce the template arguments.)

NOTE: why？

NOTE: 关于后半段的内容，移到了C-and-C++\String\Char章节中。

draft

Converting to uppercase in C++

Convert a String In C++ To Upper Case

#include "ctype.h"
#include <algorithm>
#include <iterator>
#include <string>

namespace StringUtil
{
static void ToUpper(std::string& S)
{
    std::transform(S.begin(), S.end(), S.begin(), ::toupper);
}
}