[c++] Convert a String In C++ To Upper Case

How could one convert a string to upper case. The examples I have found from googling only have to deal with chars.

The following works for me.

#include <algorithm>
void  toUpperCase(std::string& str)
{
    std::transform(str.begin(), str.end(), str.begin(), ::toupper);
}

int main()
{
   std::string str = "hello";
   toUpperCase(&str);
}

std::string str = "STriNg oF mIxID CasE lETteRS"

C++ 11

• Using for_each

  std::for_each(str.begin(), str.end(), [](char & c){ c = ::toupper(c); });

• Using transform

  std::transform(str.begin(), str.end(), str.begin(), ::toupper);

C++ (Winodws Only)

_strupr_s(str, str.length());

C++ (Using Boost Library)

boost::to_upper_copy(str)

If you only want to capitalize, try this function.

#include <iostream>


using namespace std;

string upper(string text){
    string upperCase;
    for(int it : text){
        if(it>96&&it<123){
            upperCase += char(it-32);
        }else{
            upperCase += char(it);
        }
    }
    return upperCase;
}

int main() {
    string text = "^_abcdfghopqrvmwxyz{|}";
    cout<<text<<"/";
    text = upper(text);
    cout<<text;
    return 0;
}

Error: Range-based 'for' loops are not allowed in C++98 mode

Short solution using C++11 and toupper().

for (auto & c: str) c = toupper(c);

This c++ function always returns the upper case string...

#include <locale> 
#include <string>
using namespace std; 
string toUpper (string str){
    locale loc; 
    string n; 
    for (string::size_type i=0; i<str.length(); ++i)
        n += toupper(str[i], loc);
    return n;
}

#include <string>
#include <locale>

std::string str = "Hello World!";
auto & f = std::use_facet<std::ctype<char>>(std::locale());
f.toupper(str.data(), str.data() + str.size());

This will perform better than all the answers that use the global toupper function, and is presumably what boost::to_upper is doing underneath.

This is because ::toupper has to look up the locale - because it might've been changed by a different thread - for every invocation, whereas here only the call to locale() has this penalty. And looking up the locale generally involves taking a lock.

This also works with C++98 after you replace the auto, use of the new non-const str.data(), and add a space to break the template closing (">>" to "> >") like this:

std::use_facet<std::ctype<char> > & f = 
    std::use_facet<std::ctype<char> >(std::locale());
f.toupper(const_cast<char *>(str.data()), str.data() + str.size());

The answer of @dirkgently is very inspiring, but I want to emphasize that due to the concern as is shown below,

Like all other functions from , the behavior of std::toupper 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
_{Reference: std::toupper}

As the standard does not specify if plain char is signed or unsigned[1], the correct usage of std::toupper should be:

#include <algorithm>
#include <cctype>
#include <iostream>
#include <iterator>
#include <string>

void ToUpper(std::string& input)
{
    std::for_each(std::begin(input), std::end(input), [](char& c) {
        c = static_cast<char>(std::toupper(static_cast<unsigned char>(c)));
    });
}

int main()
{
    std::string s{ "Hello world!" };
    std::cout << s << std::endl;
    ::ToUpper(s);
    std::cout << s << std::endl;

    return 0;
}

Output:

Hello world!
HELLO WORLD!

Using Boost.Text, which will work for Unicode text

boost::text::text t = "Hello World";
boost::text::text uppered;
boost::text::to_title(t, std::inserter(uppered, uppered.end()));
std::string newstr = uppered.extract();

Without using any libraries:

std::string YourClass::Uppercase(const std::string & Text)
{
    std::string UppperCaseString;
    UppperCaseString.reserve(Text.size());
    for (std::string::const_iterator it=Text.begin(); it<Text.end(); ++it)
    {
        UppperCaseString.push_back(((0x60 < *it) && (*it < 0x7B)) ? (*it - static_cast<char>(0x20)) : *it);
    }
    return UppperCaseString;
}

This problem is vectorizable with SIMD for the ASCII character set.

Speedup comparisons:

Preliminary testing with x86-64 gcc 5.2 -O3 -march=native on a Core2Duo (Merom). The same string of 120 characters (mixed lowercase and non-lowercase ASCII), converted in a loop 40M times (with no cross-file inlining, so the compiler can't optimize away or hoist any of it out of the loop). Same source and dest buffers, so no malloc overhead or memory/cache effects: data is hot in L1 cache the whole time, and we're purely CPU-bound.

• boost::to_upper_copy<char*, std::string>(): 198.0s. Yes, Boost 1.58 on Ubuntu 15.10 is really this slow. I profiled and single-stepped the asm in a debugger, and it's really, really bad: there's a dynamic_cast of a locale variable happening per character!!! (dynamic_cast takes multiple calls to strcmp). This happens with LANG=C and with LANG=en_CA.UTF-8.

  I didn't test using a RangeT other than std::string. Maybe the other form of to_upper_copy optimizes better, but I think it will always new/malloc space for the copy, so it's harder to test. Maybe something I did differs from a normal use-case, and maybe normally stopped g++ can hoist the locale setup stuff out of the per-character loop. My loop reading from a std::string and writing to a char dstbuf[4096] makes sense for testing.

• loop calling glibc toupper: 6.67s (not checking the int result for potential multi-byte UTF-8, though. This matters for Turkish.)

• ASCII-only loop: 8.79s (my baseline version for the results below.) Apparently a table-lookup is faster than a cmov, with the table hot in L1 anyway.
• ASCII-only auto-vectorized: 2.51s. (120 chars is half way between worst case and best case, see below)
• ASCII-only manually vectorized: 1.35s

See also this question about toupper() being slow on Windows when a locale is set.

I was shocked that Boost is an order of magnitude slower than the other options. I double-checked that I had -O3 enabled, and even single-stepped the asm to see what it was doing. It's almost exactly the same speed with clang++ 3.8. It has huge overhead inside the per-character loop. The perf record / report result (for the cycles perf event) is:

  32.87%  flipcase-clang-  libstdc++.so.6.0.21   [.] _ZNK10__cxxabiv121__vmi_class_type_info12__do_dyncastElNS_17__class_type_info10__sub_kindEPKS1_PKvS4_S6_RNS1_16
  21.90%  flipcase-clang-  libstdc++.so.6.0.21   [.] __dynamic_cast                                                                                                 
  16.06%  flipcase-clang-  libc-2.21.so          [.] __GI___strcmp_ssse3                                                                                            
   8.16%  flipcase-clang-  libstdc++.so.6.0.21   [.] _ZSt9use_facetISt5ctypeIcEERKT_RKSt6locale                                                                     
   7.84%  flipcase-clang-  flipcase-clang-boost  [.] _Z16strtoupper_boostPcRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE                                   
   2.20%  flipcase-clang-  libstdc++.so.6.0.21   [.] strcmp@plt                                                                                                     
   2.15%  flipcase-clang-  libstdc++.so.6.0.21   [.] __dynamic_cast@plt                                                                                             
   2.14%  flipcase-clang-  libstdc++.so.6.0.21   [.] _ZNKSt6locale2id5_M_idEv                                                                                       
   2.11%  flipcase-clang-  libstdc++.so.6.0.21   [.] _ZNKSt6locale2id5_M_idEv@plt                                                                                   
   2.08%  flipcase-clang-  libstdc++.so.6.0.21   [.] _ZNKSt5ctypeIcE10do_toupperEc                                                                                  
   2.03%  flipcase-clang-  flipcase-clang-boost  [.] _ZSt9use_facetISt5ctypeIcEERKT_RKSt6locale@plt                                                                 
   0.08% ...

Autovectorization

Gcc and clang will only auto-vectorize loops when the iteration count is known ahead of the loop. (i.e. search loops like plain-C implementation of strlen won't autovectorize.)

Thus, for strings small enough to fit in cache, we get a significant speedup for strings ~128 chars long from doing strlen first. This won't be necessary for explicit-length strings (like C++ std::string).

// char, not int, is essential: otherwise gcc unpacks to vectors of int!  Huge slowdown.
char ascii_toupper_char(char c) {
    return ('a' <= c && c <= 'z') ? c^0x20 : c;    // ^ autovectorizes to PXOR: runs on more ports than paddb
}

// gcc can only auto-vectorize loops when the number of iterations is known before the first iteration.  strlen gives us that
size_t strtoupper_autovec(char *dst, const char *src) {
    size_t len = strlen(src);
    for (size_t i=0 ; i<len ; ++i) {
        dst[i] = ascii_toupper_char(src[i]);  // gcc does the vector range check with psubusb / pcmpeqb instead of pcmpgtb
    }
    return len;
}

Any decent libc will have an efficient strlen that's much faster than looping a byte at a time, so separate vectorized strlen and toupper loops are faster.

Baseline: a loop that checks for a terminating 0 on the fly.

Times for 40M iterations, on a Core2 (Merom) 2.4GHz. gcc 5.2 -O3 -march=native. (Ubuntu 15.10). dst != src (so we make a copy), but they don't overlap (and aren't nearby). Both are aligned.

• 15 char string: baseline: 1.08s. autovec: 1.34s
• 16 char string: baseline: 1.16s. autovec: 1.52s
• 127 char string: baseline: 8.91s. autovec: 2.98s // non-vector cleanup has 15 chars to process
• 128 char string: baseline: 9.00s. autovec: 2.06s
• 129 char string: baseline: 9.04s. autovec: 2.07s // non-vector cleanup has 1 char to process

Some results are a bit different with clang.

The microbenchmark loop that calls the function is in a separate file. Otherwise it inlines and strlen() gets hoisted out of the loop, and it runs dramatically faster, esp. for 16 char strings (0.187s).

This has the major advantage that gcc can auto-vectorize it for any architecture, but the major disadvantage that it's slower for the usually-common case of small strings.

So there are big speedups, but compiler auto-vectorization doesn't make great code, esp. for cleanup of the last up-to-15 characters.

Manual vectorization with SSE intrinsics:

Based on my case-flip function that inverts the case of every alphabetic character. It takes advantage of the "unsigned compare trick", where you can do low < a && a <= high with a single unsigned comparison by range shifting, so that any value less than low wraps to a value that's greater than high. (This works if low and high aren't too far apart.)

SSE only has a signed compare-greater, but we can still use the "unsigned compare" trick by range-shifting to the bottom of the signed range: Subtract 'a'+128, so the alphabetic characters range from -128 to -128+25 (-128+'z'-'a')

Note that adding 128 and subtracting 128 are the same thing for 8bit integers. There's nowhere for the carry to go, so it's just xor (carryless add), flipping the high bit.

#include <immintrin.h>

__m128i upcase_si128(__m128i src) {
    // The above 2 paragraphs were comments here
    __m128i rangeshift = _mm_sub_epi8(src, _mm_set1_epi8('a'+128));
    __m128i nomodify   = _mm_cmpgt_epi8(rangeshift, _mm_set1_epi8(-128 + 25));  // 0:lower case   -1:anything else (upper case or non-alphabetic).  25 = 'z' - 'a'

    __m128i flip  = _mm_andnot_si128(nomodify, _mm_set1_epi8(0x20));            // 0x20:lcase    0:non-lcase

    // just mask the XOR-mask so elements are XORed with 0 instead of 0x20
    return          _mm_xor_si128(src, flip);
    // it's easier to xor with 0x20 or 0 than to AND with ~0x20 or 0xFF
}

Given this function that works for one vector, we can call it in a loop to process a whole string. Since we're already targeting SSE2, we can do a vectorized end-of-string check at the same time.

We can also do much better for the "cleanup" of the last up-to-15 bytes left over after doing vectors of 16B: upper-casing is idempotent, so re-processing some input bytes is fine. We do an unaligned load of the last 16B of the source, and store it into the dest buffer overlapping the last 16B store from the loop.

The only time this doesn't work is when the whole string is under 16B: Even when dst=src, non-atomic read-modify-write is not the same thing as not touching some bytes at all, and can break multithreaded code.

We have a scalar loop for that, and also to get src aligned. Since we don't know where the terminating 0 will be, an unaligned load from src might cross into the next page and segfault. If we need any bytes in an aligned 16B chunk, it's always safe to load the whole aligned 16B chunk.

Full source: in a github gist.

// FIXME: doesn't always copy the terminating 0.
// microbenchmarks are for this version of the code (with _mm_store in the loop, instead of storeu, for Merom).
size_t strtoupper_sse2(char *dst, const char *src_begin) {
    const char *src = src_begin;
    // scalar until the src pointer is aligned
    while ( (0xf & (uintptr_t)src) && *src ) {
        *(dst++) = ascii_toupper(*(src++));
    }

    if (!*src)
        return src - src_begin;

    // current position (p) is now 16B-aligned, and we're not at the end
    int zero_positions;
    do {
        __m128i sv = _mm_load_si128( (const __m128i*)src );
        // TODO: SSE4.2 PCMPISTRI or PCMPISTRM version to combine the lower-case and '\0' detection?

        __m128i nullcheck = _mm_cmpeq_epi8(_mm_setzero_si128(), sv);
        zero_positions = _mm_movemask_epi8(nullcheck);
        // TODO: unroll so the null-byte check takes less overhead
        if (zero_positions)
            break;

        __m128i upcased = upcase_si128(sv);   // doing this before the loop break lets gcc realize that the constants are still in registers for the unaligned cleanup version.  But it leads to more wasted insns in the early-out case

        _mm_storeu_si128((__m128i*)dst, upcased);
        //_mm_store_si128((__m128i*)dst, upcased);  // for testing on CPUs where storeu is slow
        src += 16;
        dst += 16;
    } while(1);

    // handle the last few bytes.  Options: scalar loop, masked store, or unaligned 16B.
    // rewriting some bytes beyond the end of the string would be easy,
    // but doing a non-atomic read-modify-write outside of the string is not safe.
    // Upcasing is idempotent, so unaligned potentially-overlapping is a good option.

    unsigned int cleanup_bytes = ffs(zero_positions) - 1;  // excluding the trailing null
    const char* last_byte = src + cleanup_bytes;  // points at the terminating '\0'

    // FIXME: copy the terminating 0 when we end at an aligned vector boundary
    // optionally special-case cleanup_bytes == 15: final aligned vector can be used.
    if (cleanup_bytes > 0) {
        if (last_byte - src_begin >= 16) {
            // if src==dest, this load overlaps with the last store:  store-forwarding stall.  Hopefully OOO execution hides it
            __m128i sv = _mm_loadu_si128( (const __m128i*)(last_byte-15) ); // includes the \0
            _mm_storeu_si128((__m128i*)(dst + cleanup_bytes - 15), upcase_si128(sv));
        } else {
            // whole string less than 16B
            // if this is common, try 64b or even 32b cleanup with movq / movd and upcase_si128
#if 1
            for (unsigned int i = 0 ; i <= cleanup_bytes ; ++i) {
                dst[i] = ascii_toupper(src[i]);
            }
#else
            // gcc stupidly auto-vectorizes this, resulting in huge code bloat, but no measurable slowdown because it never runs
            for (int i = cleanup_bytes - 1 ;  i >= 0 ; --i) {
                dst[i] = ascii_toupper(src[i]);
            }
#endif
        }
    }

    return last_byte - src_begin;
}

Times for 40M iterations, on a Core2 (Merom) 2.4GHz. gcc 5.2 -O3 -march=native. (Ubuntu 15.10). dst != src (so we make a copy), but they don't overlap (and aren't nearby). Both are aligned.

• 15 char string: baseline: 1.08s. autovec: 1.34s. manual: 1.29s
• 16 char string: baseline: 1.16s. autovec: 1.52s. manual: 0.335s
• 31 char string: manual: 0.479s
• 127 char string: baseline: 8.91s. autovec: 2.98s. manual: 0.925s
• 128 char string: baseline: 9.00s. autovec: 2.06s. manual: 0.931s
• 129 char string: baseline: 9.04s. autovec: 2.07s. manual: 1.02s

(Actually timed with _mm_store in the loop, not _mm_storeu, because storeu is slower on Merom even when the address is aligned. It's fine on Nehalem and later. I've also left the code as-is for now, instead of fixing the failure to copy the terminating 0 in some cases, because I don't want to re-time everything.)

So for short strings longer than 16B, this is dramatically faster than auto-vectorized. Lengths one-less-than-a-vector-width don't present a problem. They might be a problem when operating in-place, because of a store-forwarding stall. (But note that it's still fine to process our own output, rather than the original input, because toupper is idempotent).

There's a lot of scope for tuning this for different use-cases, depending on what the surrounding code wants, and the target microarchitecture. Getting the compiler to emit nice code for the cleanup portion is tricky. Using ffs(3) (which compiles to bsf or tzcnt on x86) seems to be good, but obviously that bit needs a re-think since I noticed a bug after writing up most of this answer (see the FIXME comments).

Vector speedups for even smaller strings can be obtained with movq or movd loads/stores. Customize as necessary for your use-case.

UTF-8:

We can detect when our vector has any bytes with the high bit set, and in that case fall back to a scalar utf-8-aware loop for that vector. The dst point can advance by a different amount than the src pointer, but once we get back to an aligned src pointer, we'll still just do unaligned vector stores to dst.

For text that's UTF-8, but mostly consists of the ASCII subset of UTF-8, this can be good: high performance in the common case with correct behaviour in all cases. When there's a lot of non-ASCII, it will probably be worse than staying in the scalar UTF-8 aware loop all the time, though.

Making English faster at the expense of other languages is not a future-proof decision if the downside is significant.

Locale-aware:

In the Turkish locale (tr_TR), the correct result from toupper('i') is 'I' (U0130), not 'I' (plain ASCII). See Martin Bonner's comments on a question about tolower() being slow on Windows.

We can also check for an exception-list and fallback to scalar there, like for multi-byte UTF8 input characters.

With this much complexity, SSE4.2 PCMPISTRM or something might be able to do a lot of our checks in one go.

My solution (clearing 6th bit for alpha):

#include <ctype.h>

inline void toupper(char* str)
{
    while (str[i]) {
        if (islower(str[i]))
            str[i] &= ~32; // Clear bit 6 as it is what differs (32) between Upper and Lowercases
        i++;
    }
}

#include <algorithm>
#include <string>

std::string str = "Hello World";
std::transform(str.begin(), str.end(),str.begin(), ::toupper);

template<size_t size>
char* toupper(char (&dst)[size], const char* src) {
    // generate mapping table once
    static char maptable[256];
    static bool mapped;
    if (!mapped) {
        for (char c = 0; c < 256; c++) {
            if (c >= 'a' && c <= 'z')
                maptable[c] = c & 0xdf;
            else
                maptable[c] = c;
        }
        mapped = true;
    }

    // use mapping table to quickly transform text
    for (int i = 0; *src && i < size; i++) {
        dst[i] = maptable[*(src++)];
    }
    return dst;
}

string StringToUpper(string strToConvert)
{
   for (std::string::iterator p = strToConvert.begin(); strToConvert.end() != p; ++p)
       *p = toupper(*p);

   return p;
}

Or,

string StringToUpper(string strToConvert)
{
    std::transform(strToConvert.begin(), strToConvert.end(), strToConvert.begin(), ::toupper);

    return strToConvert;
}

typedef std::string::value_type char_t;

char_t up_char( char_t ch )
{
    return std::use_facet< std::ctype< char_t > >( std::locale() ).toupper( ch );
}

std::string toupper( const std::string &src )
{
    std::string result;
    std::transform( src.begin(), src.end(), std::back_inserter( result ), up_char );
    return result;
}

const std::string src  = "test test TEST";

std::cout << toupper( src );

The faster one if you use only ASCII characters:

for(i=0;str[i]!=0;i++)
  if(str[i]<='z' && str[i]>='a')
    str[i]+='A'-'a';

Please note that this code run faster but only works on ASCII and is not an "abstract" solution.

Extended version for other UTF8 alphabets:

...
if(str[i]<='z' && str[i]>='a') //is latin
    str[i]+='A'-'a';
else if(str[i]<='?' && str[i]>='?') //cyrillic
    str[i]+='?'-'?'
else if(str[i]<='?' && str[i]>='a') //greek
    str[i]+='O'-'?'
//etc...

If you need full UNICODE solutions or more conventional and abstract solutions, go for other answers and work with methods of C++ strings.

struct convert {
   void operator()(char& c) { c = toupper((unsigned char)c); }
};

// ... 
string uc_str;
for_each(uc_str.begin(), uc_str.end(), convert());

Note: A couple of problems with the top solution:

21.5 Null-terminated sequence utilities

The contents of these headers shall be the same as the Standard C Library headers <ctype.h>, <wctype.h>, <string.h>, <wchar.h>, and <stdlib.h> [...]

• Which means that the cctype members may well be macros not suitable for direct consumption in standard algorithms.

• Another problem with the same example is that it does not cast the argument or verify that this is non-negative; this is especially dangerous for systems where plain char is signed. (The reason being: if this is implemented as a macro it will probably use a lookup table and your argument indexes into that table. A negative index will give you UB.)

not sure there is a built in function. Try this:

Include either the ctype.h OR cctype libraries, as well as the stdlib.h as part of the preprocessor directives.

string StringToUpper(string strToConvert)
{//change each element of the string to upper case
   for(unsigned int i=0;i<strToConvert.length();i++)
   {
      strToConvert[i] = toupper(strToConvert[i]);
   }
   return strToConvert;//return the converted string
}

string StringToLower(string strToConvert)
{//change each element of the string to lower case
   for(unsigned int i=0;i<strToConvert.length();i++)
   {
      strToConvert[i] = tolower(strToConvert[i]);
   }
   return strToConvert;//return the converted string
}

//works for ASCII -- no clear advantage over what is already posted...

std::string toupper(const std::string & s)
{
    std::string ret(s.size(), char());
    for(unsigned int i = 0; i < s.size(); ++i)
        ret[i] = (s[i] <= 'z' && s[i] >= 'a') ? s[i]-('a'-'A') : s[i];
    return ret;
}

Do you have ASCII or International characters in strings?

If it's the latter case, "uppercasing" is not that simple, and it depends on the used alphabet. There are bicameral and unicameral alphabets. Only bicameral alphabets have different characters for upper and lower case. Also, there are composite characters, like Latin capital letter 'DZ' (\u01F1 'DZ') which use the so called title case. This means that only the first character (D) gets changed.

I suggest you look into ICU, and difference between Simple and Full Case Mappings. This might help:

http://userguide.icu-project.org/transforms/casemappings

ALL of these solutions on this page are harder than they need to be.

Do this

RegName = "SomE StRing That you wAnt ConvErTed";
NameLength = RegName.Size();
for (int forLoop = 0; forLoop < NameLength; ++forLoop)
{
     RegName[forLoop] = tolower(RegName[forLoop]);
}

RegName is your string. Get your string size don't use string.size() as your actual tester, very messy and can cause issues. then. the most basic for loop.

remember string size returns the delimiter too so use < and not <= in your loop test.

output will be: some string that you want converted

As long as you are fine with ASCII-only and you can provide a valid pointer to RW memory, there is a simple and very effective one-liner in C:

void strtoupper(char* str)
{ 
    while (*str) *(str++) = toupper((unsigned char)*str);
}

This is especially good for simple strings like ASCII identifiers which you want to normalize into the same character-case. You can then use the buffer to construct a std:string instance.

I use this solution. I know you're not supposed to modify that data area.... but I think that's mostly for buffer overrun bugs and null character.... upper casing things isn't the same.

void to_upper(const std::string str) {
    std::string::iterator it;
    int i;
    for ( i=0;i<str.size();++i ) {
        ((char *)(void *)str.data())[i]=toupper(((char *)str.data())[i]);
    }
}

std::string value;
for (std::string::iterator p = value.begin(); value.end() != p; ++p)
    *p = toupper(*p);

try the toupper() function (#include <ctype.h>). it accepts characters as arguments, strings are made up of characters, so you'll have to iterate over each individual character that when put together comprise the string

If you are only concerned with 8 bit characters (which all other answers except Milan Babuškov assume as well) you can get the fastest speed by generating a look-up table at compile time using metaprogramming. On ideone.com this runs 7x faster than the library function and 3x faster than a hand written version (http://ideone.com/sb1Rup). It is also customizeable through traits with no slow down.

template<int ...Is>
struct IntVector{
using Type = IntVector<Is...>;
};

template<typename T_Vector, int I_New>
struct PushFront;
template<int ...Is, int I_New>
struct PushFront<IntVector<Is...>,I_New> : IntVector<I_New,Is...>{};

template<int I_Size, typename T_Vector = IntVector<>>
struct Iota : Iota< I_Size-1, typename PushFront<T_Vector,I_Size-1>::Type> {};
template<typename T_Vector>
struct Iota<0,T_Vector> : T_Vector{};

template<char C_In>
struct ToUpperTraits {
    enum { value = (C_In >= 'a' && C_In <='z') ? C_In - ('a'-'A'):C_In };
};

template<typename T>
struct TableToUpper;
template<int ...Is>
struct TableToUpper<IntVector<Is...>>{
    static char at(const char in){
        static const char table[] = {ToUpperTraits<Is>::value...};
        return table[in];
    }
};

int tableToUpper(const char c){
    using Table = TableToUpper<typename Iota<256>::Type>;
    return Table::at(c);
}

with use case:

std::transform(in.begin(),in.end(),out.begin(),tableToUpper);

For an in depth (many page) decription of how it works allow me to shamelessly plug my blog: http://metaporky.blogspot.de/2014/07/part-4-generating-look-up-tables-at.html

Use a lambda.

std::string s("change my case");

std::locale locale;
auto to_upper = [&locale] (char ch) { return std::use_facet<std::ctype<char>>(locale).toupper(ch); };

std::transform(s.begin(), s.end(), s.begin(), to_upper);

You can simply use this in C++17

for(auto i : str) putchar(toupper(i));

Here is the latest code with C++11

std::string cmd = "Hello World";
for_each(cmd.begin(), cmd.end(), [](char& in){ in = ::toupper(in); });

Based on Kyle_the_hacker's -----> answer with my extras.

Ubuntu

In terminal List all locales
locale -a

Install all locales
sudo apt-get install -y locales locales-all

Compile main.cpp
$ g++ main.cpp

Run compiled program
$ ./a.out

Results

Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë
Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë
ZOË SALDAÑA PLAYED IN LA MALDICIÓN DEL PADRE CARDONA. ËÈÑ ?O ÓÓCHLOË
ZOË SALDAÑA PLAYED IN LA MALDICIÓN DEL PADRE CARDONA. ËÈÑ ?O ÓÓCHLOË
zoë saldaña played in la maldición del padre cardona. ëèñ a? óóchloë
zoë saldaña played in la maldición del padre cardona. ëèñ a? óóchloë

Windows

In cmd run VCVARS developer tools
"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"

Compile main.cpp
> cl /EHa main.cpp /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /std:c++17 /DYNAMICBASE "kernel32.lib" "user32.lib" "gdi32.lib" "winspool.lib" "comdlg32.lib" "advapi32.lib" "shell32.lib" "ole32.lib" "oleaut32.lib" "uuid.lib" "odbc32.lib" "odbccp32.lib" /MTd

Compilador de optimización de C/C++ de Microsoft (R) versión 19.27.29111 para x64
(C) Microsoft Corporation. Todos los derechos reservados.

main.cpp
Microsoft (R) Incremental Linker Version 14.27.29111.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj
kernel32.lib
user32.lib
gdi32.lib
winspool.lib
comdlg32.lib
advapi32.lib
shell32.lib
ole32.lib
oleaut32.lib
uuid.lib
odbc32.lib
odbccp32.lib

Run main.exe
>main.exe

Results

Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë
Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë
ZOË SALDAÑA PLAYED IN LA MALDICIÓN DEL PADRE CARDONA. ËÈÑ ?O ÓÓCHLOË
ZOË SALDAÑA PLAYED IN LA MALDICIÓN DEL PADRE CARDONA. ËÈÑ ?O ÓÓCHLOË
zoë saldaña played in la maldición del padre cardona. ëèñ a? óóchloë
zoë saldaña played in la maldición del padre cardona. ëèñ a? óóchloë

The code - main.cpp

This code was only tested on Windows x64 and Ubuntu Linux x64.

/*
 * Filename: c:\Users\x\Cpp\main.cpp
 * Path: c:\Users\x\Cpp
 * Filename: /home/x/Cpp/main.cpp
 * Path: /home/x/Cpp
 * Created Date: Saturday, October 17th 2020, 10:43:31 pm
 * Author: Joma
 *
 * No Copyright 2020
 */


#include <iostream>
#include <set>
#include <string>
#include <locale>

// WINDOWS
#if (_WIN32)
#include <Windows.h>
#include <conio.h>
#define WINDOWS_PLATFORM 1
#define DLLCALL STDCALL
#define DLLIMPORT _declspec(dllimport)
#define DLLEXPORT _declspec(dllexport)
#define DLLPRIVATE
#define NOMINMAX

//EMSCRIPTEN
#elif defined(__EMSCRIPTEN__)
#include <emscripten/emscripten.h>
#include <emscripten/bind.h>
#include <unistd.h>
#include <termios.h>
#define EMSCRIPTEN_PLATFORM 1
#define DLLCALL
#define DLLIMPORT
#define DLLEXPORT __attribute__((visibility("default")))
#define DLLPRIVATE __attribute__((visibility("hidden")))

// LINUX - Ubuntu, Fedora, , Centos, Debian, RedHat
#elif (__LINUX__ || __gnu_linux__ || __linux__ || __linux || linux)
#define LINUX_PLATFORM 1
#include <unistd.h>
#include <termios.h>
#define DLLCALL CDECL
#define DLLIMPORT
#define DLLEXPORT __attribute__((visibility("default")))
#define DLLPRIVATE __attribute__((visibility("hidden")))
#define CoTaskMemAlloc(p) malloc(p)
#define CoTaskMemFree(p) free(p)

//ANDROID
#elif (__ANDROID__ || ANDROID)
#define ANDROID_PLATFORM 1
#define DLLCALL
#define DLLIMPORT
#define DLLEXPORT __attribute__((visibility("default")))
#define DLLPRIVATE __attribute__((visibility("hidden")))

//MACOS
#elif defined(__APPLE__)
#include <unistd.h>
#include <termios.h>
#define DLLCALL
#define DLLIMPORT
#define DLLEXPORT __attribute__((visibility("default")))
#define DLLPRIVATE __attribute__((visibility("hidden")))
#include "TargetConditionals.h"
#if TARGET_OS_IPHONE && TARGET_IPHONE_SIMULATOR
#define IOS_SIMULATOR_PLATFORM 1
#elif TARGET_OS_IPHONE
#define IOS_PLATFORM 1
#elif TARGET_OS_MAC
#define MACOS_PLATFORM 1
#else

#endif

#endif



typedef std::string String;
typedef std::wstring WString;

#define EMPTY_STRING u8""s
#define EMPTY_WSTRING L""s

using namespace std::literals::string_literals;

class Strings
{
public:
    static String WideStringToString(const WString& wstr)
    {
        if (wstr.empty())
        {
            return String();
        }
        size_t pos;
        size_t begin = 0;
        String ret;

#if WINDOWS_PLATFORM
        int size;
        pos = wstr.find(static_cast<wchar_t>(0), begin);
        while (pos != WString::npos && begin < wstr.length())
        {
            WString segment = WString(&wstr[begin], pos - begin);
            size = WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, &segment[0], segment.size(), NULL, 0, NULL, NULL);
            String converted = String(size, 0);
            WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, &segment[0], segment.size(), &converted[0], converted.size(), NULL, NULL);
            ret.append(converted);
            ret.append({ 0 });
            begin = pos + 1;
            pos = wstr.find(static_cast<wchar_t>(0), begin);
        }
        if (begin <= wstr.length())
        {
            WString segment = WString(&wstr[begin], wstr.length() - begin);
            size = WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, &segment[0], segment.size(), NULL, 0, NULL, NULL);
            String converted = String(size, 0);
            WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, &segment[0], segment.size(), &converted[0], converted.size(), NULL, NULL);
            ret.append(converted);
        }
#elif LINUX_PLATFORM || MACOS_PLATFORM || EMSCRIPTEN_PLATFORM
        size_t size;
        pos = wstr.find(static_cast<wchar_t>(0), begin);
        while (pos != WString::npos && begin < wstr.length())
        {
            WString segment = WString(&wstr[begin], pos - begin);
            size = wcstombs(nullptr, segment.c_str(), 0);
            String converted = String(size, 0);
            wcstombs(&converted[0], segment.c_str(), converted.size());
            ret.append(converted);
            ret.append({ 0 });
            begin = pos + 1;
            pos = wstr.find(static_cast<wchar_t>(0), begin);
        }
        if (begin <= wstr.length())
        {
            WString segment = WString(&wstr[begin], wstr.length() - begin);
            size = wcstombs(nullptr, segment.c_str(), 0);
            String converted = String(size, 0);
            wcstombs(&converted[0], segment.c_str(), converted.size());
            ret.append(converted);
        }
#else
        static_assert(false, "Unknown Platform");
#endif
        return ret;
    }

    static WString StringToWideString(const String& str)
    {
        if (str.empty())
        {
            return WString();
        }

        size_t pos;
        size_t begin = 0;
        WString ret;
#ifdef WINDOWS_PLATFORM
        int size = 0;
        pos = str.find(static_cast<char>(0), begin);
        while (pos != std::string::npos) {
            std::string segment = std::string(&str[begin], pos - begin);
            std::wstring converted = std::wstring(segment.size() + 1, 0);
            size = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, &segment[0], segment.size(), &converted[0], converted.length());
            converted.resize(size);
            ret.append(converted);
            ret.append({ 0 });
            begin = pos + 1;
            pos = str.find(static_cast<char>(0), begin);
        }
        if (begin < str.length()) {
            std::string segment = std::string(&str[begin], str.length() - begin);
            std::wstring converted = std::wstring(segment.size() + 1, 0);
            size = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, segment.c_str(), segment.size(), &converted[0], converted.length());
            converted.resize(size);
            ret.append(converted);
        }

#elif LINUX_PLATFORM || MACOS_PLATFORM || EMSCRIPTEN_PLATFORM
        size_t size;
        pos = str.find(static_cast<char>(0), begin);
        while (pos != String::npos)
        {
            String segment = String(&str[begin], pos - begin);
            WString converted = WString(segment.size(), 0);
            size = mbstowcs(&converted[0], &segment[0], converted.size());
            converted.resize(size);
            ret.append(converted);
            ret.append({ 0 });
            begin = pos + 1;
            pos = str.find(static_cast<char>(0), begin);
        }
        if (begin < str.length())
        {
            String segment = String(&str[begin], str.length() - begin);
            WString converted = WString(segment.size(), 0);
            size = mbstowcs(&converted[0], &segment[0], converted.size());
            converted.resize(size);
            ret.append(converted);
        }
#else
        static_assert(false, "Unknown Platform");
#endif
        return ret;
    }


    static WString ToUpper(const WString& data)
    {
        WString result = data;
        auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());

        f.toupper(&result[0], &result[0] + result.size());
        return result;
    }

    static String  ToUpper(const String& data)
    {
        return WideStringToString(ToUpper(StringToWideString(data)));
    }

    static WString ToLower(const WString& data)
    {
        WString result = data;
        auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());
        f.tolower(&result[0], &result[0] + result.size());
        return result;
    }

    static String ToLower(const String& data)
    {
        return WideStringToString(ToLower(StringToWideString(data)));
    }

};

enum class ConsoleTextStyle
{
    DEFAULT = 0,
    BOLD = 1,
    FAINT = 2,
    ITALIC = 3,
    UNDERLINE = 4,
    SLOW_BLINK = 5,
    RAPID_BLINK = 6,
    REVERSE = 7,
};

enum class ConsoleForeground
{
    DEFAULT = 39,
    BLACK = 30,
    DARK_RED = 31,
    DARK_GREEN = 32,
    DARK_YELLOW = 33,
    DARK_BLUE = 34,
    DARK_MAGENTA = 35,
    DARK_CYAN = 36,
    GRAY = 37,
    DARK_GRAY = 90,
    RED = 91,
    GREEN = 92,
    YELLOW = 93,
    BLUE = 94,
    MAGENTA = 95,
    CYAN = 96,
    WHITE = 97
};

enum class ConsoleBackground
{
    DEFAULT = 49,
    BLACK = 40,
    DARK_RED = 41,
    DARK_GREEN = 42,
    DARK_YELLOW = 43,
    DARK_BLUE = 44,
    DARK_MAGENTA = 45,
    DARK_CYAN = 46,
    GRAY = 47,
    DARK_GRAY = 100,
    RED = 101,
    GREEN = 102,
    YELLOW = 103,
    BLUE = 104,
    MAGENTA = 105,
    CYAN = 106,
    WHITE = 107
};

class Console
{
private:
    static void EnableVirtualTermimalProcessing()
    {
#if defined WINDOWS_PLATFORM
        HANDLE hOut = GetStdHandle(STD_OUTPUT_HANDLE);
        DWORD dwMode = 0;
        GetConsoleMode(hOut, &dwMode);
        if (!(dwMode & ENABLE_VIRTUAL_TERMINAL_PROCESSING))
        {
            dwMode |= ENABLE_VIRTUAL_TERMINAL_PROCESSING;
            SetConsoleMode(hOut, dwMode);
        }
#endif
    }

    static void ResetTerminalFormat()
    {
        std::cout << u8"\033[0m";
    }

    static void SetVirtualTerminalFormat(ConsoleForeground foreground, ConsoleBackground background, std::set<ConsoleTextStyle> styles)
    {
        String format = u8"\033[";
        format.append(std::to_string(static_cast<int>(foreground)));
        format.append(u8";");
        format.append(std::to_string(static_cast<int>(background)));
        if (styles.size() > 0)
        {
            for (auto it = styles.begin(); it != styles.end(); ++it)
            {
                format.append(u8";");
                format.append(std::to_string(static_cast<int>(*it)));
            }
        }
        format.append(u8"m");
        std::cout << format;
    }
public:
    static void Clear()
    {

#ifdef WINDOWS_PLATFORM
        std::system(u8"cls");
#elif LINUX_PLATFORM || defined MACOS_PLATFORM
        std::system(u8"clear");
#elif EMSCRIPTEN_PLATFORM
        emscripten::val::global()["console"].call<void>(u8"clear");
#else
        static_assert(false, "Unknown Platform");
#endif
    }

    static void Write(const String& s, ConsoleForeground foreground = ConsoleForeground::DEFAULT, ConsoleBackground background = ConsoleBackground::DEFAULT, std::set<ConsoleTextStyle> styles = {})
    {
#ifndef EMSCRIPTEN_PLATFORM
        EnableVirtualTermimalProcessing();
        SetVirtualTerminalFormat(foreground, background, styles);
#endif
        String str = s;
#ifdef WINDOWS_PLATFORM
        WString unicode = Strings::StringToWideString(str);
        WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), unicode.c_str(), static_cast<DWORD>(unicode.length()), nullptr, nullptr);
#elif defined LINUX_PLATFORM || defined MACOS_PLATFORM || EMSCRIPTEN_PLATFORM
        std::cout << str;
#else
        static_assert(false, "Unknown Platform");
#endif

#ifndef EMSCRIPTEN_PLATFORM
        ResetTerminalFormat();
#endif
    }

    static void WriteLine(const String& s, ConsoleForeground foreground = ConsoleForeground::DEFAULT, ConsoleBackground background = ConsoleBackground::DEFAULT, std::set<ConsoleTextStyle> styles = {})
    {
        Write(s, foreground, background, styles);
        std::cout << std::endl;
    }

    static void Write(const WString& s, ConsoleForeground foreground = ConsoleForeground::DEFAULT, ConsoleBackground background = ConsoleBackground::DEFAULT, std::set<ConsoleTextStyle> styles = {})
    {
#ifndef EMSCRIPTEN_PLATFORM
        EnableVirtualTermimalProcessing();
        SetVirtualTerminalFormat(foreground, background, styles);
#endif
        WString str = s;

#ifdef WINDOWS_PLATFORM
        WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), str.c_str(), static_cast<DWORD>(str.length()), nullptr, nullptr);
#elif LINUX_PLATFORM || MACOS_PLATFORM || EMSCRIPTEN_PLATFORM
        std::cout << Strings::WideStringToString(str);
#else
        static_assert(false, "Unknown Platform");
#endif

#ifndef EMSCRIPTEN_PLATFORM
        ResetTerminalFormat();
#endif
    }

    static void WriteLine(const WString& s, ConsoleForeground foreground = ConsoleForeground::DEFAULT, ConsoleBackground background = ConsoleBackground::DEFAULT, std::set<ConsoleTextStyle> styles = {})
    {
        Write(s, foreground, background, styles);
        std::cout << std::endl;
    }

    static void WriteLine()
    {
        std::cout << std::endl;
    }

    static void Pause()
    {
        char c;
        do
        {
            c = getchar();
            std::cout << "Press Key " << std::endl;
        } while (c != 64);
        std::cout << "KeyPressed" << std::endl;
    }

    static int PauseAny(bool printWhenPressed = false, ConsoleForeground foreground = ConsoleForeground::DEFAULT, ConsoleBackground background = ConsoleBackground::DEFAULT, std::set<ConsoleTextStyle> styles = {})
    {
        int ch;
#ifdef WINDOWS_PLATFORM
        ch = _getch();
#elif LINUX_PLATFORM || MACOS_PLATFORM || EMSCRIPTEN_PLATFORM
        struct termios oldt, newt;
        tcgetattr(STDIN_FILENO, &oldt);
        newt = oldt;
        newt.c_lflag &= ~(ICANON | ECHO);
        tcsetattr(STDIN_FILENO, TCSANOW, &newt);
        ch = getchar();
        tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
#else
        static_assert(false, "Unknown Platform");
#endif
        if (printWhenPressed)
        {
            Console::Write(String(1, ch), foreground, background, styles);
        }
        return ch;
    }
};



int main()
{
    std::locale::global(std::locale(u8"en_US.UTF-8"));
    String dataStr = u8"Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë";
    WString dataWStr = L"Zoë Saldaña played in La maldición del padre Cardona. ëèñ a? óóChloë";
    std::string locale = u8"";
    //std::string locale = u8"de_DE.UTF-8";
    //std::string locale = u8"en_US.UTF-8";
    Console::WriteLine(dataStr);
    Console::WriteLine(dataWStr);
    dataStr = Strings::ToUpper(dataStr);
    dataWStr = Strings::ToUpper(dataWStr);
    Console::WriteLine(dataStr);
    Console::WriteLine(dataWStr);
    dataStr = Strings::ToLower(dataStr);
    dataWStr = Strings::ToLower(dataWStr);
    Console::WriteLine(dataStr);
    Console::WriteLine(dataWStr);
    
    
    Console::WriteLine(u8"Press any key to exit"s, ConsoleForeground::DARK_GRAY);
    Console::PauseAny();

    return 0;
}