c++ - 初始化 stringstream.str( a_value ) 和 stringstream << a_value 之间的区别

Question

考虑:

std::string        s_a, s_b;

std::stringstream  ss_1, ss_2;

// at this stage:
//     ss_1 and ss_2 have been used and are now in some strange state
//     s_a and s_b contain non-white space words

ss_1.str( std::string() );
ss_1.clear();

ss_1 << s_a;
ss_1 << s_b;

// ss_1.str().c_str() is now the concatenation of s_a and s_b, 
//                    <strike>with</strike> without space between them

ss_2.str( s_a );
ss_2.clear();

// ss_2.str().c_str() is now s_a

ss_2 << s_b;  // line ***

// ss_2.str().c_str() the value of s_a is over-written by s_b 
// 
// Replacing line *** above with "ss_2 << ss_2.str() << " " << s_b;"
//                    results in ss_2 having the same content as ss_1.

问题:

1. stringstream.str(a_value); 有什么区别？和字符串流 << a_value;并且，具体来说，为什么第一个不允许通过 << 进行连接，但第二个允许？

2. 为什么 ss_1 会自动在 s_a 和 s_b 之间获得空白，但是我们是否需要在可能的行中显式添加空格替换行***: ss_2 << ss_2.str() << " " << s_b; ?

Answer 1

您遇到的问题是因为 std::stringstream默认构造为 ios_base::openmode mode = ios_base::in|ios_base::out这是一个非附加模式。

您对这里的输出模式感兴趣(即:ios_base::openmode mode = ios_base::out)

std::basic_stringbuf::str(const std::basic_string<CharT, Traits, Allocator>& s)以两种不同的方式运行，具体取决于 openmode :

1. mode & ios_base::ate == false :(即:非附加输出流):

   str将设置 pptr() == pbase() ，以便后续输出将覆盖从 s 复制的字符

2. mode & ios_base::ate == true :(即:附加输出流):

   str将设置 pptr() == pbase() + s.size() ，以便后续输出将附加到从 s 复制的最后一个字符

(注意这个追加模式是c++11以后的新模式)

更多详情可查看here .

如果您想要 附加 行为，请创建您的 stringstream与 ios_base::ate :

std::stringstream ss(std::ios_base::out | std::ios_base::ate)

这里是简单的示例应用程序:

#include <iostream>
#include <sstream>

void non_appending()
{
    std::stringstream ss;
    std::string s = "hello world";

    ss.str(s);
    std::cout << ss.str() << std::endl;

    ss << "how are you?";
    std::cout << ss.str() << std::endl;
}

void appending()
{
    std::stringstream ss(std::ios_base::out | std::ios_base::ate);
    std::string s = "hello world";

    ss.str(s);
    std::cout << ss.str() << std::endl;

    ss << "how are you?";
    std::cout << ss.str() << std::endl;
}

int main()
{
    non_appending();
    appending();

    exit(0);
}

这将以上述两种不同的方式输出:

hello world
how are you?
hello world
hello worldhow are you?