How to convert an instance of std::string to lower case

Boost provides a string algorithm for this:

#include <boost/algorithm/string.hpp> std::string str = "HELLO, WORLD!"; boost::algorithm::to_lower(str); // modifies str 

Or, for non-in-place:

#include <boost/algorithm/string.hpp> const std::string str = "HELLO, WORLD!"; const std::string lower_str = boost::algorithm::to_lower_copy(str); 

2025 edit:

ICU has found an actual contender for "the way things are done" in ztd.text, an actual submission to the C++ committee that seems to do quite well if you trust the author's round of tests.

None of what I said years ago is wrong, and much of it is still very much on point: The C++ standard is still ill equipped to handle encodings. But it seems a ztd.text based solution is closer than an ICU based one (although ICU is still the most widely used third party library for the purpose).

Anyway, here's my original answer...

tl;dr

Use the ICU library. If you don't, your conversion routine will break silently on cases you are probably not even aware of existing.

First you have to answer a question: What is the encoding of your std::string? Is it ISO-8859-1? Or perhaps ISO-8859-8? Or Windows Codepage 1252? Does whatever you're using to convert upper-to-lowercase know that? (Or does it fail miserably for characters over 0x7f?)

If you are using UTF-8 (the only sane choice among the 8-bit encodings) with std::string as container, you are already deceiving yourself if you believe you are still in control of things. You are storing a multibyte character sequence in a container that is not aware of the multibyte concept, and neither are most of the operations you can perform on it! Even something as simple as .substr() could result in invalid (sub-) strings because you split in the middle of a multibyte sequence.

As soon as you try something like std::toupper( 'ß' ), or std::tolower( 'Σ' ) in any encoding, you are in trouble. Because 1), the standard only ever operates on one character at a time, so it simply cannot turn ß into SS as would be correct. And 2), the standard only ever operates on one character at a time, so it cannot decide whether Σ is in the middle of a word (where σ would be correct), or at the end (ς). Another example would be std::tolower( 'I' ), which should yield different results depending on the locale -- virtually everywhere you would expect i, but in Turkey ı (LATIN SMALL LETTER DOTLESS I) is the correct answer (which, again, is more than one byte in UTF-8 encoding).

So, any case conversion that works on a character at a time, or worse, a byte at a time, is broken by design. This includes all the std:: variants in existence at this time.

Then there is the point that the standard library, for what it is capable of doing, is depending on which locales are supported on the machine your software is running on... and what do you do if your target locale is among the not supported on your client's machine?

So what you are really looking for is a string class that is capable of dealing with all this correctly, and that is not any of the std::basic_string<> variants.

(C++11 note: std::u16string and std::u32string are better, but still not perfect. C++20 brought std::u8string, but all these do is specify the encoding. In many other respects they still remain ignorant of Unicode mechanics, like normalization, collation, ...)

While Boost looks nice, API wise, Boost.Locale is basically a wrapper around ICU. If Boost is compiled with ICU support... if it isn't, Boost.Locale is limited to the locale support compiled for the standard library.

And believe me, getting Boost to compile with ICU can be a real pain sometimes. (There are no pre-compiled binaries for Windows that include ICU, so you'd have to supply them together with your application, and that opens a whole new can of worms...)

So personally I would recommend getting full Unicode support straight from the horse's mouth and using the ICU library directly:

#include <unicode/unistr.h> #include <unicode/ustream.h> #include <unicode/locid.h> #include <iostream> int main() { /* "Odysseus" */ char const * someString = u8"ΟΔΥΣΣΕΥΣ"; icu::UnicodeString someUString( someString, "UTF-8" ); // Setting the locale explicitly here for completeness. // Usually you would use the user-specified system locale, // which *does* make a difference (see ı vs. i above). std::cout << someUString.toLower( "el_GR" ) << "\n"; std::cout << someUString.toUpper( "el_GR" ) << "\n"; return 0; } 

Compile (with G++ in this example):

g++ -Wall example.cpp -licuuc -licuio 

This gives:

ὀδυσσεύς 

Note that the Σ<->σ conversion in the middle of the word, and the Σ<->ς conversion at the end of the word. No <algorithm>-based solution can give you that.

Another approach using range based for loop with reference variable

string test = "Hello World"; for(auto& c : test) { c = tolower(c); } cout<<test<<endl; 

Using range-based for loop of C++11 a simpler code would be :

#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); } 

If the string contains UTF-8 characters outside of the ASCII range, then boost::algorithm::to_lower will not convert those. Better use boost::locale::to_lower when UTF-8 is involved. See http://www.boost.org/doc/libs/1_51_0/libs/locale/doc/html/conversions.html

This is a follow-up to Stefan Mai's response: if you'd like to place the result of the conversion in another string, you need to pre-allocate its storage space prior to calling std::transform. Since STL stores transformed characters at the destination iterator (incrementing it at each iteration of the loop), the destination string will not be automatically resized, and you risk memory stomping.

#include <string> #include <algorithm> #include <iostream> int main (int argc, char* argv[]) { std::string sourceString = "Abc"; std::string destinationString; // Allocate the destination space destinationString.resize(sourceString.size()); // Convert the source string to lower case // storing the result in destination string std::transform(sourceString.begin(), sourceString.end(), destinationString.begin(), ::tolower); // Output the result of the conversion std::cout << sourceString << " -> " << destinationString << std::endl; } 

Simplest way to convert string into loweercase without bothering about std namespace is as follows

1:string with/without spaces

#include <algorithm> #include <iostream> #include <string> using namespace std; int main(){ string str; getline(cin,str); //------------function to convert string into lowercase--------------- transform(str.begin(), str.end(), str.begin(), ::tolower); //-------------------------------------------------------------------- cout<<str; return 0; } 

2:string without spaces

#include <algorithm> #include <iostream> #include <string> using namespace std; int main(){ string str; cin>>str; //------------function to convert string into lowercase--------------- transform(str.begin(), str.end(), str.begin(), ::tolower); //-------------------------------------------------------------------- cout<<str; return 0; } 

I wrote this simple helper function:

#include <locale> // tolower string to_lower(string s) { for(char &c : s) c = tolower(c); return s; } 

Usage:

string s = "TEST"; cout << to_lower("HELLO WORLD"); // output: "hello word" cout << to_lower(s); // won't change the original variable. 

My own template functions which performs upper / lower case.

#include <string> #include <algorithm> // // Lowercases string // template <typename T> std::basic_string<T> lowercase(const std::basic_string<T>& s) { std::basic_string<T> s2 = s; std::transform(s2.begin(), s2.end(), s2.begin(), [](const T v){ return static_cast<T>(std::tolower(v)); }); return s2; } // // Uppercases string // template <typename T> std::basic_string<T> uppercase(const std::basic_string<T>& s) { std::basic_string<T> s2 = s; std::transform(s2.begin(), s2.end(), s2.begin(), [](const T v){ return static_cast<T>(std::toupper(v)); }); return s2; } 

std::ctype::tolower() from the standard C++ Localization library will correctly do this for you. Here is an example extracted from the tolower reference page

#include <locale> #include <iostream> int main () { std::locale::global(std::locale("en_US.utf8")); std::wcout.imbue(std::locale()); std::wcout << "In US English UTF-8 locale:\n"; auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale()); std::wstring str = L"HELLo, wORLD!"; std::wcout << "Lowercase form of the string '" << str << "' is "; f.tolower(&str[0], &str[0] + str.size()); std::wcout << "'" << str << "'\n"; } 

An alternative to Boost is POCO (pocoproject.org).

POCO provides two variants:

1. The first variant makes a copy without altering the original string.
2. The second variant changes the original string in place.
   "In Place" versions always have "InPlace" in the name.
   

Both versions are demonstrated below:

#include "Poco/String.h" using namespace Poco; std::string hello("Stack Overflow!"); // Copies "STACK OVERFLOW!" into 'newString' without altering 'hello.' std::string newString(toUpper(hello)); // Changes newString in-place to read "stack overflow!" toLowerInPlace(newString); 

Since none of the answers mentioned the upcoming Ranges library, which is available in the standard library since C++20, and currently separately available on GitHub as range-v3, I would like to add a way to perform this conversion using it.

To modify the string in-place:

str |= action::transform([](unsigned char c){ return std::tolower(c); }); 

To generate a new string:

auto new_string = original_string | view::transform([](unsigned char c){ return std::tolower(c); }); 

(Don't forget to #include <cctype> and the required Ranges headers.)

Note: the use of unsigned char as the argument to the lambda is inspired by cppreference, which states:

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; } 

There is a way to convert upper case to lower WITHOUT doing if tests, and it's pretty straight-forward. The isupper() function/macro's use of clocale.h should take care of problems relating to your location, but if not, you can always tweak the UtoL[] to your heart's content.

Given that C's characters are really just 8-bit ints (ignoring the wide character sets for the moment) you can create a 256 byte array holding an alternative set of characters, and in the conversion function use the chars in your string as subscripts into the conversion array.

Instead of a 1-for-1 mapping though, give the upper-case array members the BYTE int values for the lower-case characters. You may find islower() and isupper() useful here.

The code looks like this...

#include <clocale> static char UtoL[256]; // ---------------------------------------------------------------------------- void InitUtoLMap() { for (int i = 0; i < sizeof(UtoL); i++) { if (isupper(i)) { UtoL[i] = (char)(i + 32); } else { UtoL[i] = i; } } } // ---------------------------------------------------------------------------- char *LowerStr(char *szMyStr) { char *p = szMyStr; // do conversion in-place so as not to require a destination buffer while (*p) { // szMyStr must be null-terminated *p = UtoL[*p]; p++; } return szMyStr; } // ---------------------------------------------------------------------------- int main() { time_t start; char *Lowered, Upper[128]; InitUtoLMap(); strcpy(Upper, "Every GOOD boy does FINE!"); Lowered = LowerStr(Upper); return 0; } 

This approach will, at the same time, allow you to remap any other characters you wish to change.

This approach has one huge advantage when running on modern processors, there is no need to do branch prediction as there are no if tests comprising branching. This saves the CPU's branch prediction logic for other loops, and tends to prevent pipeline stalls.

Some here may recognize this approach as the same one used to convert EBCDIC to ASCII.

On microsoft platforms you can use the strlwr family of functions: http://msdn.microsoft.com/en-us/library/hkxwh33z.aspx

// crt_strlwr.c // compile with: /W3 // This program uses _strlwr and _strupr to create // uppercase and lowercase copies of a mixed-case string. #include <string.h> #include <stdio.h> int main( void ) { char string[100] = "The String to End All Strings!"; char * copy1 = _strdup( string ); // make two copies char * copy2 = _strdup( string ); _strlwr( copy1 ); // C4996 _strupr( copy2 ); // C4996 printf( "Mixed: %s\n", string ); printf( "Lower: %s\n", copy1 ); printf( "Upper: %s\n", copy2 ); free( copy1 ); free( copy2 ); } 

Here's a macro technique if you want something simple:

#define STRTOLOWER(x) std::transform (x.begin(), x.end(), x.begin(), ::tolower) #define STRTOUPPER(x) std::transform (x.begin(), x.end(), x.begin(), ::toupper) #define STRTOUCFIRST(x) std::transform (x.begin(), x.begin()+1, x.begin(), ::toupper); std::transform (x.begin()+1, x.end(), x.begin()+1,::tolower) 

However, note that @AndreasSpindler's comment on this answer still is an important consideration, however, if you're working on something that isn't just ASCII characters.

Is there an alternative which works 100% of the time?

No

There are several questions you need to ask yourself before choosing a lowercasing method.

1. How is the string encoded? plain ASCII? UTF-8? some form of extended ASCII legacy encoding?
2. What do you mean by lower case anyway? Case mapping rules vary between languages! Do you want something that is localised to the users locale? do you want something that behaves consistently on all systems your software runs on? Do you just want to lowercase ASCII characters and pass through everything else?
3. What libraries are available?

Once you have answers to those questions you can start looking for a soloution that fits your needs. There is no one size fits all that works for everyone everywhere!

C++ doesn't have tolower or toupper methods implemented for std::string, but it is available for char. One can easily read each char of string, convert it into required case and put it back into string. A sample code without using any third party library:

#include<iostream> int main(){ std::string str = std::string("How ARe You"); for(char &ch : str){ ch = std::tolower(ch); } std::cout<<str<<std::endl; return 0; } 

For character based operation on string : For every character in string

// tolower example (C++) #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 (std::string::size_type i=0; i<str.length(); ++i) std::cout << std::tolower(str[i],loc); return 0; } 

For more information: http://www.cplusplus.com/reference/locale/tolower/

Try this function :)

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; } 

Have a look at the excellent c++17 cpp-unicodelib (GitHub). It's single-file and header-only.

 #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; 

Output

Je suis là! je suis là! 

An explanation of how this solution works:

string test = "Hello World"; for(auto& c : test) { c = tolower(c); } 

Explanation:

for(auto& c : test) is a range-based for loop of the kind
for ( range_declaration:range_expression)loop_statement:

1. range_declaration: auto& c
   Here the auto specifier is used for for automatic type deduction. So the type gets deducted from the variables initializer.

2. range_expression: test
   The range in this case are the characters of string test.

The characters of the string test are available as a reference inside the for loop through identifier c.

Use fplus::to_lower_case() from fplus library.

Search to_lower_case in fplus API Search

Example:

fplus::to_lower_case(std::string("ABC")) == std::string("abc"); 

Google's absl library has absl::AsciiStrToLower / absl::AsciiStrToUpper

Since you are using std::string, you are using c++. If using c++11 or higher, this doesn't need anything fancy. If words is vector<string>, then:

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

Doesn't have strange exceptions. Might want to use w_char's but otherwise this should do it all in place.

For a different perspective, there is a very common use case which is to perform locale neutral case folding on Unicode strings. For this case, it is possible to get good case folding performance when you realize that the set of foldable characters is finite and relatively small (< 2000 Unicode code points). It happens to work very well with a generated perfect hash (guaranteed zero collisions) can be used to convert every input character to its lowercase equivalent.

With UTF-8, you do have to be conscientious of multi-byte characters and iterate accordingly. However, UTF-8 has fairly simple encoding rules that make this operation efficient.

For more details, including links to the relevant parts of the Unicode standard and a perfect hash generator, see my answer here, to the question How to achieve unicode-agnostic case insensitive comparison in C++.

#include <iostream> class LowerCaseString : public std::string { public: LowerCaseString(std::string anyCase) : std::string(ToLower(anyCase)){} LowerCaseString(const char* anyCase) : std::string(ToLower(std::string(anyCase))) {} private: std::string& ToLower(std::string& anyCase) { std::string& lowerCase = anyCase; for (auto& c : lowerCase) { c = std::tolower(c); } return lowerCase; } std::string ToLower(const std::string& anyCase) { std::string lowerCase = anyCase; for (auto& c : lowerCase) { c = std::tolower(c); } return lowerCase; } }; int main() { LowerCaseString str(std::string("TEST")); std::cout << str << std::endl; LowerCaseString str1 = std::string("TEST1"); std::cout << str1 << std::endl; LowerCaseString str2 = "TEST2"; std::cout << str2 << std::endl; } 

The derived class from std::string can offer all functionality of std::string and convert a std::string or char array to lower case std::string.

If your looking for a C implementation (usable with C++) that handles UTF-8 quite well and is also very small, you could also have a look here:

How to uppercase/lowercase UTF-8 characters in C++?

These C-functions can be easily wrapped for use with std::string.

I'm not saying this is the most robust way, after all, all the problems with std::string will remain, but it could be helpful in some use cases.

Two simple (non optimized) functions that use this C-code to implement lcase/ucase in C++ with std::string:

inline std::string lcase(std::string in) { const Utf8Char *s_utf8 = reinterpret_cast<const Utf8Char *>(in.c_str()); Utf8Char *new_s = Utf8StrMakeLwrUtf8Str(s_utf8); if (new_s == nullptr) { std::cerr << "ERR: cannot allocate enough memory for new string, returning original std::string" << std::endl; return in; } else { std::string out(reinterpret_cast<char *>(new_s)); return out; } } inline std::string ucase(std::string in) { const Utf8Char *s_utf8 = reinterpret_cast<const Utf8Char *>(in.c_str()); Utf8Char *new_s = Utf8StrMakeUprUtf8Str(s_utf8); if (new_s == nullptr) { std::cerr << "ERR: cannot allocate enough memory for new string, returning original std::string" << std::endl; return in; } else { std::string out(reinterpret_cast<char *>(new_s)); return out; } } 

Code Snippet

#include<bits/stdc++.h> using namespace std; int main () { ios::sync_with_stdio(false); string str="String Convert\n"; for(int i=0; i<str.size(); i++) { str[i] = tolower(str[i]); } cout<<str<<endl; return 0; } 

Add some optional libraries for ASCII string to_lower, both of which are production level and with micro-optimizations, which is expected to be faster than the existed answers here(TODO: add benchmark result).

Facebook's Folly:

void toLowerAscii(char* str, size_t length) 

Google's Abseil:

void AsciiStrToLower(std::string* s);