Can't convert string to lower case

Question

I'm trying to convert string to lower case, here is my program:

#include <iostream>
#include <algorithm>
#include <string>
using namespace std;

int main(){
    string a;
    getline(cin, a);
    transform(a.begin(), a.end(), a.begin(), ::tolower);
    cout << a;
}

When I build the program, I get the warning:

C4244 '=': conversion from 'int' to 'char', possible loss of data

I know tolower return int, but how can I change it to char, can someone help me with this? Thanks!

Also, `tolower` should be inside std namespace, not global scope, that is the c function. — Mansoor, Jan 30 '21 at 16:36
https://stackoverflow.com/questions/313970/how-to-convert-stdstring-to-lower-case contains some very good answers to the question you didn't really ask, but might have wanted to ask. — Thomas, Jan 30 '21 at 16:39

Kevin · Accepted Answer · 2021-01-30T20:33:54.350

Your use of tolower has undefined behavior! The cppreference page on std::tolower tells you exactly what to do to fix it:

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;
}

So your code should look like this:

transform(a.begin(), a.end(), a.begin(), [](unsigned char c) { return std::tolower(c); });

As for your warning, it's due to the warning level you have set for your compiler. You get around it you can explicitly cast the result:

transform(a.begin(), a.end(), a.begin(), [](unsigned char c) {
    return static_cast<char>(std::tolower(c));
});

Can't convert string to lower case

1 Answers1