How to Convert a C++ String to Uppercase

问题:

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)

回答1:

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 can thus not 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.

回答2:

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

回答3:

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

回答4:

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));
}

回答5:

You can also use the function from code below to convert it to Upper-case.

#include<iostream>
#include<cstring>

using namespace std;

//Function for Converting Lower-Case to Upper-Case
void fnConvertUpper(char str[], char* des)
{
    int i;
    char c[1 + 1];
    memset(des, 0, sizeof(des)); //memset the variable before using it.
    for (i = 0; i <= strlen(str); i++) {
        memset(c, 0, sizeof(c));
        if (str[i] >= 97 && str[i] <= 122) {
            c[0] = str[i] - 32;    // here we are storing the converted value into 'c' variable, hence we are memseting it inside the for loop, so before storing a new value we are clearing the old value in 'c'.
        } else {
            c[0] = str[i];
        }
        strncat(des, &c[0], 1);
    }
}

int main()
{
    char str[20];  //Source Variable
    char des[20];  //Destination Variable

    //memset the variables before using it so as to clear any values which it contains,it can also be a junk value.
    memset(str, 0, sizeof(str));  
    memset(des, 0, sizeof(des));

    cout << "Enter the String (Enter First Name) : ";
    cin >> str; //getting the value from the user and storing it into Source variable.

    fnConvertUpper(str, des); //Now passing the source variable(which has Lower-Case value) along with destination variable, once the function is successfully executed the destination variable will contain the value in Upper-Case

    cout << "\nThe String in Uppercase = " << des << "\n"; //now print the destination variable to check the Converted Value.
}