[英]Difference between initializations stringstream.str( a_value ) and stringstream << a_value
考慮:
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.
問題:
stringstream.str(a_value)之間的區別是什么? 和stringstream << a_value; 而且,具體來說,為什么第一個不允許通過<<但第二個進行連接?
為什么ss_1會自動在s_a和save之間獲得空白,但是我們是否需要在行中明確添加可以替換行***的
ss_2 << ss_2.str() << " " << s_b;
:
ss_2 << ss_2.str() << " " << s_b;
?
建議你閱讀stringstream參考: http : //en.cppreference.com/w/cpp/io/basic_stringstream
std::stringstream::str()
替換基礎字符串的內容
operator<<
數據插入流中。
您遇到的問題是因為默認情況下使用ios_base::openmode mode = ios_base::in|ios_base::out
構造std::stringstream
ios_base::openmode mode = ios_base::in|ios_base::out
這是一種非附加模式。
你對這里的輸出模式感興趣(即: ios_base::openmode mode = ios_base::out
)
std::basic_stringbuf::str(const std::basic_string<CharT, Traits, Allocator>& s)
以兩種不同的方式運行,具體取決於openmode
:
mode & ios_base::ate == false
:(即: 非附加輸出流):
str
將設置pptr() == pbase()
,以便后續輸出將覆蓋從s復制的字符
mode & ios_base::ate == true
:(即: 附加輸出流):
str
將設置pptr() == pbase() + s.size()
,以便后續輸出將附加到從s復制的最后一個字符
(注意,這個追加模式是新的,因為c ++ 11)
更多細節可以在這里找到。
如果您想要追加行為,請使用ios_base::ate
創建stringstream
:
std::stringstream ss(std::ios_base::out | std::ios_base::ate)
這里簡單的示例應用:
#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);
}
這將以上述兩種不同的方式輸出:
hello world
how are you?
hello world
hello worldhow are you?
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