string的size/length、resize/reverse和capacity

野性酷女 2022-08-08 04:30 16阅读 0赞

# **函数原型**    #

**reserve**

原型:

#include <string>
     void reserve( size_type size = 0 );

函数reserve()将字符串的容量设置为至少size. **如果size指定的数值要小于当前字符串中的字符数****(亦即size < this→size()****), 容量将被设置为可以恰好容纳字符的数值**. reserve()以线性时间([linear time][])运行.

## [resize][] ##

原型:

#include <string>
     void resize( size_type size, char val = char() );

函数resize()将字符串的尺寸设置为size. 如果val被指定, 则新创建的元素都将被初始化为val. 此函数以线性时间([linear time][])运行.

## [capacity][] ##

原型:

#include <string>
     size_type capacity() const;

函数capacity()返回string预先保留的最大元素数量.

string类型中**size()和length()**分别返回string中含有的字符个数、string的字符串长度；对于某个string来说，**两者的返回值是一样**的。

对于如下的代码：

int main() {
    	//section A
    	string str;
    	getline(cin, str);
    	cout << "size: " << str.size() << endl;
    	cout << "length: " << str.length() << endl;
    	cout << "capacity: " << str.capacity() << endl;
    
    	//section B
    	/*str.resize(2);
    	cout << endl;
    	cout << str << endl;
    	cout << "size: " << str.size() << endl;
    	cout << "length: " << str.length() << endl;
    	cout << "capacity: " << str.capacity() << endl;*/
    
    	//section C
    	str.reserve(2);
    	cout << endl;
    	cout << str << endl;
    	cout << "size: " << str.size() << endl;
    	cout << "length: " << str.length() << endl;
    	cout << "capacity: " << str.capacity() << endl;
    }

如果运行section A 和 section B部分代码，结果如图所示：

![SouthEast][]

如果运行section A 和section　C部分代码，结果如图所示：

![SouthEast 1][]

可以看出，resize后将string类型的str的空间变为了resize的数值；而reserve后，**如果size指定的数值要小于当前字符串中的字符数****(亦即size < this→size()****), 容量将被设置为可以恰好容纳字符的数值**

[linear time]: http://huycwork.blog.163.com/blog/static/13675199920102271193431/
[resize]: http://www.cppreference.com/wiki/string/resize
[capacity]: http://www.cppreference.com/wiki/string/capacity
[SouthEast]: /images/20220805/d41eb23a46054f8daed4564c1f553745.png
[SouthEast 1]: /images/20220805/9d12cb7d6c124731bdcbc0daba127409.png