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I need to convert a string in C++ to full upper case. I've been searching for a while and found one way to do it:

#include <iostream> #include <algorithm> #include <string> using namespace std; int main() { string input; cin >> input; transform(input.begin(), input.end(), input.begin(), toupper); cout << input; return 0; }

Unfortunately this did not work and I received this error message:

no matching function for call to 'transform(std::basic_string::iterator, std::basic_string::iterator, std::basic_string::iterator,

I've tried other methods that also did not work. This was the closest to working.

So what I'm asking is what I am doing wrong. Maybe my syntax is bad or I need to include something. I am not sure.

I got most of my info here: http://www.cplusplus.com/forum/beginner/75634/ (last two posts)

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7 Answers 7

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60

You need to put a double colon before toupper:

transform(input.begin(), input.end(), input.begin(), ::toupper);

Explanation:

There are two different toupper functions:

  1. toupper in the global namespace (accessed with ::toupper), which comes from C.

  2. toupper in the std namespace (accessed with std::toupper) which has multiple overloads and thus cannot be simply referenced with a name only. You have to explicitly cast it to a specific function signature in order to be referenced, but the code for getting a function pointer looks ugly: static_cast<int (*)(int)>(&std::toupper)

Since you're using namespace std, when writing toupper, 2. hides 1. and is thus chosen, according to name resolution rules.

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5 Comments

@LokiAstari: The point is that you want the global toupper, not the std:: one(s), which highlights the problem with use namespace std -- it pollutes the default namespace with lots of symbols you don't want just to get one or two. If you want to import just a few symbols, you should import just those symbols, not the entire std namespace.
@LokiAstari: No -- if you remove the using namespace std; then you can use just toupper in this code and it will work (no need for the ::). Of course, you need to add using std::transform; (or stick std:: before transform) and similarly for other symbols in std that are being used.
In addition, you may need #include <ctype.h> instead of #include <cctype> to get toupper in the global namespace -- but that's independent of everything else. (<cctype> is allowed to put toupper in the global namespace, but not required to)
@leemes How can I do if I want to only first character in string to uppercase? I tried with ::toupper(input[0]) but it didn't work.
@HasanBasri toupper returns the modified character instead of updating it in place. That means, you need to assign back the result: input[0] = toupper(input[0]). (Here, I didn't put the two colons, which effectively means I want std::toupper when using namespace std).
10

Boost string algorithms:

#include <boost/algorithm/string.hpp> #include <string> std::string str = "Hello World"; boost::to_upper(str); std::string newstr = boost::to_upper_copy("Hello World");

Convert a String In C++ To Upper Case

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7

Try this small program, straight from C++ reference

#include <iostream> #include <algorithm> #include <string> #include <functional> #include <cctype> using namespace std; int main() { string s; cin >> s; std::transform(s.begin(), s.end(), s.begin(), std::ptr_fun<int, int>(std::toupper)); cout << s; return 0; }

Live demo

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std::ptr_fun() is deprecated in c++11 and removed in c++14.
6

Uppercase to Lowercase and viceversa using BitWise operators

1.

string s = "cAPsLock"; for(char &c: s) c = c | ' '; // similar to: c = tolower(c); cout << s << endl; // output: capslock 
string s = "cAPsLock"; for(char &c: s) c = c & ~' '; // similar to: c = toupper(c); cout << s << endl; // output: CAPSLOCK

PS: for more info check this link

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3

You could do:

string name = "john doe"; //or just get string from user... for(int i = 0; i < name.size(); i++) { name.at(i) = toupper(name.at(i)); }
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3 
#include <iostream> using namespace std; //function for converting string to upper string stringToUpper(string oString){ for(int i = 0; i < oString.length(); i++){ oString[i] = toupper(oString[i]); } return oString; } int main() { //use the function to convert string. No additional variables needed. cout << stringToUpper("Hello world!") << endl; return 0; }
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0

Like leemes said, you can use toupper(int). Like this:

void ToUpper(string &str) { for (auto beg = str.begin(); beg != str.end(); ++beg) { *beg = toupper(*beg); } }

It'll through in each character from str and convert it to upper. Example:

int main() { string name; cout << "Insert a name: "; cin >> name; ToUpper(name); cout << "Name in upper case: " << name << endl; }
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