Utilities for STL std::string

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Utilities for STL std::string

Lots of programmers have been familiar with some routines for string object, 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 to do something which above routines do?

Of course, std::string supplies some methods to implements some routines above. They are,

length(), get the length of the string.
substr(), get a substring of the string.
at()/operator [], get the char at 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 specified characters.
find_first_not_of(), find the first character that is not any of specified characters.
find_last_of(), find the last character that is any of specified characters.
find_last_not_of(), find the last character that is not any of specified characters.

Please refer document for more std::string's methods

Some routines are not implemented as std::string's methods, but we can find way in algorithm.h to do that. Of course, the existed 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 document for detail of 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 trimed.

Create string by repeating character or substring

If you want create a string by repeating substring, you must use 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 just to repeat 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 attend compare. Then transform all of them to lower case. At last, just compare the two lower case strings.

StartsWith and EndsWith

StartsWith

str.find(substr) == 0;

If result is true, the str starts with substr.

EndsWith

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

If result is true, the str ends with substr

There is another way to do that. Just get left substring or right substring to compare. Because I don't want to calculate if string's length is enough, so 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 use C++ way to do. So I choose std::istringstream. the class is in sstream.h.

A template function can do most excludes bool value.

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 number as true. But it cannot parse "false" as false and "true" as true. So I write 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 literal bool value.

It is possible to use a similar way to parse 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 string from other kinds of value. The class is also in sstream.h. The relative 3 functions are followed.

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();
}

Do you take note of setw and setfill? They are still flags which need an argument. std::setw allow the output thing in the stream occupy fixed width. If itself length is not enough, default uses space to fill. std::setfill is used to change the spaceholder. If you want control the alignment, there are std::left and std::right flags.

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

Split and tokenizer

I think split function should be implemented with a tokenizer. So I write a tokenizer at first. We can use find_first_of and find_first_not_of methods to get each token. Follows is nextToken method of 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 whole Tokenizer is in the source code archive. You can download it at above. All other functions are still in the source code files.