sstream

未分类

---

istringstream

typedef basic_istringstream<char> istringstream;

default (1) 
explicit istringstream (ios_base::openmode which = ios_base::in);
initialization (2)  
explicit istringstream (const string& str,
                        ios_base::openmode which = ios_base::in);
copy (3)    
istringstream (const istringstream&) = delete;
move (4)    
istringstream (istringstream&& x);

具体用法与```ifstream````差不多，都是一个输入流

ostringstream

typedef basic_ostringstream<char> ostringstream;

explicit ostringstream (ios_base::openmode which = ios_base::out);
explicit ostringstream (const string& str,
                        ios_base::openmode which = ios_base::out);

具体用法与ofstream差不多，都是一个输出流

stringstream

template < class charT,                           // basic_stringstream::char_type
           class traits = char_traits<charT>,     // basic_stringstream::traits_type
           class Alloc = allocator<charT>         // basic_stringstream::allocator_type
           > class basic_stringstream;

概念与fstream差不多，既是输入流也是输出流，但是stringstream在类型转换方法上大派用场。

stringstream::stringstream

explicit stringstream (ios_base::openmode which = ios_base::in | ios_base::out);
explicit stringstream (const string& str,
                       ios_base::openmode which = ios_base::in | ios_base::out);

example

ifstream ifs("a.txt");
string str;
stringstream ss;
int n;
while(ifs>>str){ //getline(ifs,str);
    ss.str(str);//ss.clear()仅仅是情况状态标识，非清空流中字符
    ss>>n;//以空格、回车为分隔符读取流中字符到n
    cout<<n<<endl;
}
ifs.close();

当文件a.txt涉及多种类型数据时，如string,int
张三 99
李四 98
...
可以如下读取

ifstream ifs("a.txt");
stringstream ss;
string name;
int age;
string line;
while(getline(ifs,line)){
    ss.clear();//需要先情况stringstream 缓存中的流数据，否则下一个name，age依然是上一轮的值
    ss.str(line);//将流中数据设置为line，等价于ss.str("");ss<<line;
    ss>>name>>age;//以空格、回车为分隔符读取流中数据
    cout<<name<<" "<<age<<endl;
}
ifs.close();

C语言的fscanf也可以读入数据，经过测试，4亿条数据，使用C++的getline+stringstream耗时81秒，而fscanf耗时18s，在追求效率是fscanf确实快。

#include <cstdio>
using std::fscanf;
using std::fopen;
using std::fclose;
int main(int argc, char* argv[]) {
    FILE* fptr = fopen("a.txt"."r");
    char name[80];
    int age;
    while(fscanf(fptr,"%s%d",name,&age) > 0) {
        //do sth
    }
    fclose(fptr);
    return 0;
}