将对象直接流式传输到std :: string [英] Stream object directly into a std::string
问题描述
给出某种可流式传输的类型:
Given some type that is streamable:
struct X {
int i;
friend std::ostream& operator<<(std::ostream& os, X const& x) {
return os << "X(" << x.i << ')';
}
};
我想将此附加到 std :: string
上.我可以这样实现:
I want to append this onto a std::string
. I can implement this as:
void append(std::string& s, X const& x) {
std::ostringstream os;
os << x;
s.append(os.str());
}
但是这似乎很me脚,因为我将数据写入一个流中只是为了分配一个新字符串,只是为了将其附加到另一个字符串上.有更直接的路线吗?
But this seems lame since I'm writing data into one stream just to then allocate a new string just for the purposes of appending it onto a different one. Is there a more direct route?
推荐答案
这可以通过新型标准C ++ IOStreams和语言环境:高级程序员指南和参考).
以下是它的外观草图:
#include <streambuf>
class existing_string_buf : public std::streambuf
{
public:
// Store a pointer to to_append.
explicit existing_string_buf(std::string &to_append);
virtual int_type overflow (int_type c) {
// Push here to the string to_append.
}
};
一旦您在此处充实了详细信息,就可以按以下方式使用它:
Once you flesh out the details here, you could use it as follows:
#include <iostream>
std::string s;
// Create a streambuf of the string s
existing_string_buf b(s);
// Create an ostream with the streambuf
std::ostream o(&b);
现在,您只需写入 o
,结果应显示为附加在 s
上.
Now you just write to o
, and the result should appear as appended to s
.
// This will append to s
o << 22;
修改
正如@rustyx正确指出的那样,需要重写 xsputn
来提高性能.
As @rustyx correctly notes, overriding xsputn
is required for improving performance.
完整示例
以下打印为 22
:
#include <streambuf>
#include <string>
#include <ostream>
#include <iostream>
class existing_string_buf : public std::streambuf
{
public:
// Somehow store a pointer to to_append.
explicit existing_string_buf(std::string &to_append) :
m_to_append(&to_append){}
virtual int_type overflow (int_type c) {
if (c != EOF) {
m_to_append->push_back(c);
}
return c;
}
virtual std::streamsize xsputn (const char* s, std::streamsize n) {
m_to_append->insert(m_to_append->end(), s, s + n);
return n;
}
private:
std::string *m_to_append;
};
int main()
{
std::string s;
existing_string_buf b(s);
std::ostream o(&b);
o << 22;
std::cout << s << std::endl;
}
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