C++ String

整理自：Thinking in C++, Volume 2

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1. Creating strings

To use strings you include the C++ header file <string>. The string class is also in the namespace std.

string imBlank;
string heyMom("Where are my socks?");
string standardReply = "Beamed into deep "
	"space on wide angle dispersion?";	// concat string literals
string useThisOneAgain(standardReply);

// Copy the first 8 chars:
string s4(s1, 0, 8);

// concat
string quoteMe = s4 + "that";

string source("xxx");
string s(source.begin(), source.end()); // string iterator
assert(s == source);

string okay(5, 'a'); // initialize a string with a number of copies of a single character
assert(okay == string("aaaaa"));

2. Operating on strings

2.1 Appending, inserting, and concatenating strings

#include <string>
#include <iostream>
using namespace std;

int main() {
    string bigNews("I saw Elvis in a UFO. ");
    cout << bigNews << endl;

	// How much data have we actually got?
    cout << "Size = " << bigNews.size() << endl;
    cout << "Length = " << bigNews.length() << endl; // .length() and .size() are identical
	// How much can we store without reallocating?
    cout << "Capacity = " << bigNews.capacity() << endl;

	// Insert this string in bigNews immediately
	// before bigNews[1]:
    bigNews.insert(1, " thought I");
    cout << bigNews << endl;
    cout << "Size = " << bigNews.size() << endl;
    cout << "Capacity = " << bigNews.capacity() << endl;
	
	// Make sure that there will be this much space
    bigNews.reserve(500); // 注意是 reserve 不是 reverse 
	
	// Add this to the end of the string:
    bigNews.append("I've been working too hard.");
    cout << bigNews << endl;
    cout << "Size = " << bigNews.size() << endl;
    cout << "Capacity = " << bigNews.capacity() << endl;
    
    // appends spaces if the new size is greater than the current string size or truncates the string otherwise
    bigNews.resize(10);
    cout << bigNews << endl;
    cout << "Size = " << bigNews.size() << endl;
    cout << "Capacity = " << bigNews.capacity() << endl;
}

// output: 
/* 
	I saw Elvis in a UFO.
	Size = 22
	Length = 22
	Capacity = 22
	I thought I saw Elvis in a UFO.
	Size = 32
	Capacity = 44
	I thought I saw Elvis in a UFO. I've been working too hard.
	Size = 59
	Capacity = 500
	I thought
	Size = 10
	Capacity = 500
*/

2.2 Replacing string characters

There are a number of overloaded versions of string::replace(), but the simplest one takes three arguments:

• an integer indicating where to start in the string,
• an integer indicating how many characters to eliminate from the original string,
• and the replacement string.

#include <cassert>
#include <cstddef> // For size_t
#include <string>
using namespace std;

int main() {
    string bigNews = "I thought I saw Elvis in a UFO. "
		"I have been working too hard.";
    string replacement("wig");
    string findMe("UFO");
    
	// Look in bigNews for the "UFO"
	// starting at position 0:
    size_t i = bigNews.find(findMe, 0);
	
	// Did we find the "UFO"?
    if(i != string::npos) {
    	// Replace the "UFO" with "wig":
        bigNews.replace(i, findMe.size(), replacement);
	}

	assert(bigNews == "I thought I saw Elvis in a wig. "
		"I have been working too hard.");
}

另外还有一个 generic algorithm replace():

#include <algorithm>
#include <cassert>
#include <string>
using namespace std;

int main() {
	string s("aaaXaaaXXaaXXXaXXXXaaa");
	replace(s.begin(), s.end(), 'X', 'Y');
	assert(s == "aaaYaaaYYaaYYYaYYYYaaa");
}

The replace() algorithm only works with single objects (in this case, char objects) and will not replace quoted char arrays or string objects.

2.3 Removing characters from strings

string::erase() takes two arguments:

• where to start removing characters (which defaults to 0),
• and how many to remove (which defaults to string::npos).

因为 string::npos 其实是个极大的整数，而 If you specify more characters than remain in the string, the remaining characters are all erased anyway. So calling string::npos without any arguments removes all characters in it.

3. Searching in strings

string::find() family，用法大体相同：找到了就返回 starting position of the first match，没找到就返回 string:npos。family 有：

• find()
• find_first_of()
• find_last_of()
• find_first_not_of()
• find_last_not_of()
• rfind() // find reversely

见名知意。

4. Comparing strings

C++ 的字符串是可以用 >、<、>=、<=、!=、== 来比较大小的，遵循的是字母序，比如 “a” < “b”，”aa” < “ab” 这样。

另外也可以用 string::compare()，与 java 的用法一致。

5. Strings and character traits

Observe how string is declared in the Standard C++ header file:

typedef basic_string<char> string;

template <class CharType, class Traits = char_traits<CharType>, class Allocator = allocator<CharType>>
class basic_string;

根据 MSDN: basic_string Class 的说法：

• CharType for string is char
• CharType for wstring is wchar_t
• CharType for u16string is char16_t
• CharType for u32string is char32_t

To change the way the string class treats character comparison (e.g. take uppercase and lowercase into accounts), you must supply a different char_traits<> template because that defines the behavior of the individual character comparison member functions.

书上 P111 给了一个例子。