C++ stringstream学习

转自：https://blog.csdn.net/hanjing_csdn/article/details/79922660

https://zhuanlan.zhihu.com/p/44435521

1.介绍 

当我们需要在程序中使用字符串和数字数据互相转换的时候，可以使用stringstream类，

• 通过运算符 ”<<“ 将数据传递给 stringstream 对象；
• 通过调用stringstream 类的函数str() 将对象所包含的内容赋给一个string对象；
• 可以方便的以流运算符<<将数值以各种数据(字串、数值)写入stringstream对象，且不用担心写越界等问题；

2.使用 

#include <iostream>
#include <sstream>
int main()
{
    // default constructor (input/output stream)
    std::stringstream buf1;
    buf1 << 7;//将int转换为stringstream对象
    int n = 0;
    buf1 >> n;
    std::cout << "buf1 = " << buf1.str() << " n = " << n << '\n';
 
    // input stream
    std::istringstream inbuf("-10");
    inbuf >> n;//可以将stringstream对象转换为int
    std::cout << "n = " << n << '\n';
 
    // output stream in append mode (C++11)
    std::ostringstream buf2("test", std::ios_base::ate);//如果不设置第二个参数的话，就会变为1est
    buf2 << '1';
    std::cout << buf2.str() << '\n';
}

输出：

buf1 = 7 n = 7
n = -10
test1 

转换过程：

• 数字 -> stringstream对象 -> string 
• string -> stringstream对象 -> 数字

#include <sstream>
#include <iostream>
int main()
{
    int n;
 
    std::istringstream in;  // could also use in("1 2")
    in.str("1 2");
    in >> n;
    std::cout << "after reading the first int from \"1 2\", the int is "
              << n << ", str() = \"" << in.str() << "\"\n";
 
    std::ostringstream out("1 2");
    out << 3;
    std::cout << "after writing the int '3' to output stream \"1 2\""
              << ", str() = \"" << out.str() << "\"\n";
    out << 4;
    std::cout << "after writing the int '4' to output stream \"1 2\""
              << ", str() = \"" << out.str() << "\"\n";
    out << 5;
    std::cout << "after writing the int '5' to output stream \"1 2\""
              << ", str() = \"" << out.str() << "\"\n";

    std::ostringstream ate("1 2", std::ios_base::ate);
    ate << 3;
    std::cout << "after writing the int '3' to append stream \"1 2\""
              << ", str() = \"" << ate.str() << "\"\n";
    return 0;
}

输出为：

after reading the first int from "1 2", the int is 1, str() = "1 2"
after writing the int '3' to output stream "1 2", str() = "3 2"//插入时如果不设置，默认从头开始插入
after writing the int '4' to output stream "1 2", str() = "342"
after writing the int '5' to output stream "1 2", str() = "345"
after writing the int '3' to append stream "1 2", str() = "1 23"

>>操作也默认从头开始输出，<<默认从头开始输入。（感觉好反直觉。。）

3.效率

转自：https://blog.csdn.net/mijichui2153/article/details/118154341

此链接的实验中，+=和append操作，效率都高，但是stringstream由于是类，频繁地创建和销毁代价较高，涉及到内存分配、对象构造和销毁。如果循环中需要频繁使用stringstream对象的话，可以共用一个，在使用中clear和清空：

void* test_stringstream(void * arg)
{
    stringstream oss;
    for(int i=0;i<10000;i++)
    {
        oss.clear();这仅仅置流标记
        oss.str("");/这是才是真正清空操作
        oss << i;
    }
}

stringstream不会主动释放内存，stream.str("")清除缓冲。