I want to convert a std::string to lowercase. I am aware of the function tolower(). However, in the past I have had issues with this function and it is hardly ideal anyway as using it with a std::string would require iterating over each character.
Is there an alternative which works 100% of the time?
I wrote a templated version that works with any string :
#include <type_traits> // std::decay #include <ctype.h> // std::toupper & std::tolower template <class T = void> struct farg_t { using type = T; }; template <template<typename ...> class T1, class T2> struct farg_t <T1<T2>> { using type = T2*; }; //--------------- template<class T, class T2 = typename std::decay< typename farg_t<T>::type >::type> void ToUpper(T& str) { T2 t = &str[0]; for (; *t; ++t) *t = std::toupper(*t); } template<class T, class T2 = typename std::decay< typename farg_t<T>::type >::type> void Tolower(T& str) { T2 t = &str[0]; for (; *t; ++t) *t = std::tolower(*t); } Tested with gcc compiler:
#include <iostream> #include "upove_code.h" int main() { std::string str1 = "hEllo "; char str2 [] = "wOrld"; ToUpper(str1); ToUpper(str2); std::cout << str1 << str2 << '\n'; Tolower(str1); Tolower(str2); std::cout << str1 << str2 << '\n'; return 0; } output:
>HELLO WORLD > >hello world This could be another simple version to convert uppercase to lowercase and vice versa. I used VS2017 community version to compile this source code.
#include <iostream> #include <string> using namespace std; int main() { std::string _input = "lowercasetouppercase"; #if 0 // My idea is to use the ascii value to convert char upperA = 'A'; char lowerA = 'a'; cout << (int)upperA << endl; // ASCII value of 'A' -> 65 cout << (int)lowerA << endl; // ASCII value of 'a' -> 97 // 97-65 = 32; // Difference of ASCII value of upper and lower a #endif // 0 cout << "Input String = " << _input.c_str() << endl; for (int i = 0; i < _input.length(); ++i) { _input[i] -= 32; // To convert lower to upper #if 0 _input[i] += 32; // To convert upper to lower #endif // 0 } cout << "Output String = " << _input.c_str() << endl; return 0; } Note: if there are special characters then need to be handled using condition check.
tolower()doesn't work 100% of the time. Lowercase/uppercase operations only apply to characters, and std::string is essentially an array of bytes, not characters. Plaintoloweris nice for ASCII string, but it will not lowercase a latin-1 or utf-8 string correctly. You must know string's encoding and probably decode it before you can lowercase its characters.