assign方法可以理解为先将原字符串清空，然后赋予新的值作替换。

返回类型为 string类型的引用。其常用的重载也有下列几种：

a. string& assign ( const string& str );

将str替换原字串的内容

举例：

string testassign = "Hello World";

testassign.assign("Go home");

cout<<testassign<<endl;

//打印结果为 go home

b. string& assign ( const string& str, size_t pos, size_t n );

将str的内容从位置pos起的n个字符作为原字串的新内容赋给原字串

string testassign = "Hello World";

testassign.assign("Come on!", 5, 2);

cout<<testassign<<endl;

//打印结果为 on

c. string& assign ( const char* s, size_t n );

将字符数组或者字符串的首n个字符替换原字符串内容

举例：

string testassign = "Hello World";

testassign.assign("go back to China", 7);

cout<<testassign<<endl;

//打印结果为go back

d. string& assign ( const char* s );

将字符串或者字符数组作为新内容替换原字串

举例：

string testassign = "Hello World";

char ch[20] = "go back to shanghai";

testassign.assign(ch);

cout<<testassign<<endl;

//打印结果为 go back to shanghai

e. string& assign ( size_t n, char c );

将原字串替换为n个字符c

举例：

string testassign = "Hello World";

char ch = ‘?‘;

testassign.assign(5, ch);

cout<<testassign<<endl;

//打印结果为?????

f. template <class InputIterator>   string& assign ( InputIterator first, InputIterator last );

需要include <iterator>

举例：

string testassign = "Hello World";

testassign.assign(istream_iterator<char>(cin), istream_iterator<char>());

//输入abcde

cout<<testassign<<endl;

//打印结果为 abcde

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string& assign ( const string& str );
string& assign ( const string& str, size_t pos, size_t n );
string& assign ( const char* s, size_t n );
string& assign ( const char* s );
string& assign ( size_t n, char c );
template <class InputIterator>
   string& assign ( InputIterator first, InputIterator last );

Assigns new content to the string replacing its current content.

The arguments passed to the function determine the new content:

string& assign ( const string& str );

Sets a copy of str as the new content.

string& assign ( const string& str, size_t pos, size_t n );

Sets a copy of a substring of str as the new content. The substring is the portion of str that begins at the character position pos and takes up to n characters (it takes less than n if the end of str is reached before).

string& assign ( const char * s, size_t n );

Sets as the new content a copy of the string formed by the first n characters of the array pointed by s.

string& assign ( const char * s );

Sets a copy of the string formed by the null-terminated character sequence (C string) pointed by s as the new content. The length of the character sequence is determined by the first ocurrence of a null character (as determined by traits.length(s)).

string& assign ( size_t n, char c );

Sets a string formed by a repetition of character c, n times, as the new content.

template<class InputIterator> string& assign (InputIterator first, InputIterator last);

If InputIterator is an integral type, behaves as the previous member function version, effectively setting as the new content a string formed by the repetition first times of the character equivalent to last.
In any other case, the content is set to the values of the elements that go from element referred to by iterator first to the element right before the one referred to by iterator last.

Parameters

str

Another object of class string whose content is entirely or partially copied as the new content for the string.

pos

Starting position of the substring of the string object str that forms the new content. Notice that the first position has a value of 0, not 1.
If the position passed is past the end of str, an out_of_range exception is thrown.

n

Number of characters to use for the content (i.e., its length).

s

Array with a sequence of characters.
In the third member function version, the length is determined by parameter n, even including null characters in the content.
By contrast, in the fourth member version, s is a null-terminated character, and therefore its length is determined only by the first occurrence of a null character.

c

Character value to be repeated n times to form the new content.

start

If along with last, both are integers, it is equivalent to parameter n, otherwise it is an iterator referring to the beginning of a sequence of characters.

last

If along with start, both are integers, it is equivalent to parameter c, otherwise it is an iterator referring to the past-the-end element of a sequence of characters.

Return Value

*this

Example

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// string::assign
#include <iostream>
#include <string>
using namespace std;

int main ()
{
  string str;
  string base="The quick brown fox jumps over a lazy dog.";

  // used in the same order as described above:

  str.assign(base);
  cout << str << endl;

  str.assign(base,10,9);
  cout << str << endl;         // "brown fox"

  str.assign("pangrams are cool",7);
  cout << str << endl;         // "pangram"

  str.assign("c-string");
  cout << str << endl;         // "c-string"

  str.assign(10,‘*‘);
  cout << str << endl;         // "**********"

  str.assign<int>(10,0x2D);
  cout << str << endl;         // "----------"

  str.assign(base.begin()+16,base.end()-12);
  cout << str << endl;         // "fox jumps over"

  return 0;
}