Utilities for STL std::string

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Utilities for STL std::string

Lots of programmers have been familiar with various routines for string objects, such as length, substring, find, charAt, toLowerCase, toUpperCase, trim, equalsIgnoreCase, startsWith, endsWith, parseInt, toString, split, and so on.

Now, if you are using STL and its string class std::string, how do you do something which the above routines do?

Of course, std::string supplies some methods to implement some of the routines above. They are:

• length(): get the length of the string.
• substr(): get a substring of the string.
• at()/operator []: get the char at the specified location in the string.
• find/rfind(): search a string in a forward/backward direction for a substring.
• find_first_of(): find the first character that is any of the specified characters.
• find_first_not_of(): find the first character that is not any of the specified characters.
• find_last_of(): find the last character that is any of the specified characters.
• find_last_not_of(): find the last character that is not any of the specified characters.

Please refer to the document for more std::string methods.

Some routines are not implemented as std::string methods, but we can find a way in algorithm.h to do that. Of course, the existing methods of std::string are also used to implement them.

Transform a string to upper/lower case

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

Please refer to the document for details of the std::transform function.

Trim spaces beside a string

Trim left spaces

string::iterator i;
for (i = str.begin(); i != str.end(); i++) {
    if (!isspace(*i)) {
        break;
    }
}
if (i == str.end()) {
    str.clear();
} else {
    str.erase(str.begin(), i);
}

Trim right spaces

string::iterator i;
for (i = str.end() - 1; ;i--) {
    if (!isspace(*i)) {
        str.erase(i + 1, str.end());
        break;
    }
    if (i == str.begin()) {
        str.clear();
        break;
    }
}

Trim two-sided spaces

Trim left spaces then trim right spaces. Thus two-sided spaces are trimmed.

Create string by repeating a character or substring

If you want create a string by repeating a substring, you must use a loop to implement it.

string repeat(const string& str, int n) {
    string s;
    for (int i = 0; i < n; i++) {
        s += str;
    }
    return s;
}

But if you need to just repeat a character, std::string has a constructor.

string repeat(char c, int n) {
    return string(n, c);
}

Compare ignore case

It's funny. We should copy the two strings which we are attempting to compare. Then transform all of it to lower case. At last, just compare the two lower case strings.

StartsWith and EndsWith

StartsWith

str.find(substr) == 0;

If the result is true, str starts with substr.

EndsWith

size_t i = str.rfind(substr);
return (i != string::npos) && (i == (str.length() - substr.length()));

If result is true, str ends with substr.

There is another way to do that. Just get the left substring or right substring to compare. Because I don't want to calculate if the string's length is enough, I use find and rfind to do that.

Parse number/bool from a string

For these routines, atoi, atol, and some other C functions are OK. But I want to use the C++ way to do it. So I choose std::istringstream. The class is in sstream.h.

A template function can do most, excluding bool values.

template<class T> parseString(const std::string& str) {
    T value;
    std::istringstream iss(str);
    iss >> value;
    return value;
}

The template function can parse 0 as false and other numbers as true. But it cannot parse "false" as false and "true" as true. So I wrote a special function.

template<bool>
bool parseString(const std::string& str) {
    bool value;
    std::istringstream iss(str);
    iss >> boolalpha >> value;
    return value;
}

As you saw, I pass a std::boolalpha flag to the input stream, then the input stream can recognize the literal bool value.

It is possible to use a similar way to parse a hex string. This time I should pass a std::hex flag to the stream.

template<class T> parseHexString(const std::string& str) {
    T value;
    std::istringstream iss(str);
    iss >> hex >> value;
    return value;
}

To string routines

Like parsing from string, I will use std::ostringstream to get a string from other kinds of values. The class is also in sstream.h. The relative three functions are shown here.

template<class T> std::string toString(const T& value) {
    std::ostringstream oss;
    oss << value;
    return oss.str();
}
string toString(const bool& value) {
    ostringstream oss;
    oss << boolalpha << value;
    return oss.str();
}
template<class T> std::string toHexString(const T& value, int width) {
    std::ostringstream oss;
    oss << hex;
    if (width > 0) {
        oss << setw(width) 
            << setfill('0');
    }
    oss << value;
    return oss.str();
}

Did you take note of setw and setfill? They are still flags which need an argument. std::setw allows the output thing in the stream to occupy a fixed width. If its length is not enough, by default it uses spaces to fill. std::setfill is used to change the space holder. If you want to control the alignment, there are the std::left and std::right flags.

Oh, I forgot to tell you, setw and setfill need the iomanip.h header file.

Split and tokenizer

I think the split function should be implemented with a tokenizer. So I wrote a tokenizer first. We can use the find_first_of and find_first_not_of methods to get each token. Shown below is the nextToken method of the Tokenizer class.

bool Tokenizer::nextToken(const std::string& delimiters) {
    // find the start character of the next token.
    size_t i = m_String.find_first_not_of(delimiters, m_Offset);
    if (i == string::npos) {
        m_Offset = m_String.length();
        return false;
    }

    // find the end of the token.
    size_t j = m_String.find_first_of(delimiters, i);
    if (j == string::npos) {
        m_Token = m_String.substr(i);
        m_Offset = m_String.length();
        return true;
    }

    // to intercept the token and save current position
    m_Token = m_String.substr(i, j - i);
    m_Offset = j;
    return true;
}

The complete tokenizer is available in the source code archive. You can download it from the link above. All other functions are still in the source code files.