string::substr

std::string::substr

string substr (size_t pos = 0, size_t len = npos) const;

Generate substring

Returns a newly constructed string object with its value initialized to a copy of a substring of this object.

The substring is the portion of the object that starts at character position pos and spans len characters (or until the end of the string, whichever comes first).

Parameters

pos

Position of the first character to be copied as a substring.
If this is equal to the string length, the function returns an empty string.
If this is greater than the string length, it throws out_of_range.
Note: The first character is denoted by a value of 0 (not 1).

len

Number of characters to include in the substring (if the string is shorter, as many characters as possible are used).
A value of string::npos indicates all characters until the end of the string.

size_t is an unsigned integral type (the same as member type string::size_type).

Return Value

A string object with a substring of this object.

Example

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// string::substr #include <iostream> #include <string> int main () { std::string str="We think in generalities, but we live in details."; // (quoting Alfred N. Whitehead) std::string str2 = str.substr (3,5); // "think" std::size_t pos = str.find("live"); // position of "live" in str std::string str3 = str.substr (pos); // get from "live" to the end std::cout << str2 << ' ' << str3 << '\n'; return 0; }

Edit & Run

Output:

think live in details.

Complexity

Unspecified, but generally linear in the length of the returned object.

Iterator validity

No changes.

Data races

The object is accessed.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the string.

If pos is greater than the string length, an out_of_range exception is thrown.

A bad_alloc exception is thrown if the function needs to allocate storage and fails.

substr(起始位置，長度)

#include<string>

#include<iostream>

using namespace std;

main()

{

string s("12345asdf");

string a=s.substr(0,4);    //獲得字符串s中 從第0位開始的長度爲4的字符串

cout<<a<<endl;

}

輸出結果爲:

1234

用處：：：

1.可以用在字符串的反轉中。

2.可以用在字符串的循環移位中。