基本上我想做一些相当于这样的事情：
{
std::string s(“Possibly very long user string”);
const char* mine = s.c_str();
// 'mine' will be passed along,pass(mine);
//Made-up call
s.release_data();
// 's' should not release data,but it should properly destroy itself otherwise.
}
要澄清,我确实需要摆脱std :: string：进一步走下坡路.该代码处理字符串和二进制数据,并应以相同的格式处理它.而且我想要std :: string的数据,因为它来自另一个与std :: string一起工作的代码层.
为了给我更多的观点,我想要这样做：例如我有一个异步的套接字封装,应该能够使用std :: string和二进制数据从用户的写作.两个“API”写入版本(采用std :: string或行二进制数据)内部解析为相同的(二进制)写入.我需要避免任何复制,因为字符串可能很长.
WriteId write( std::unique_ptr< std::string > strToWrite )
{
// Convert std::string data to contiguous byte storage
// that will be further passed along to other
// functions (also with the moving semantics).
// strToWrite.c_str() would be a solution to my problem
// if I could tell strToWrite to simply give up its
// ownership. Is there a way?
unique_ptr<std::vector<char> > dataToWrite= ??
//
scheduleWrite( dataToWrite );
}
void scheduledWrite( std::unique_ptr< std::vecor<char> > data)
{
…
}
std :: unique_ptr在这个例子中说明所有权转让：任何其他具有相同语义的方法对我来说都是好的.
我想知道这个具体情况的解决方案(使用std :: string char缓冲区)和字符串,流和类似的一般问题：在string,stream,std容器和缓冲区类型之间移动缓冲区的提示.
我也会赞赏提示和链接C设计方法和具体技术,当涉及传递缓冲区数据之间不同的API /类型而不复制.我提到但不使用流,因为我在这个问题上摇摇欲坠.
解决方法
How can I take ownership of std::string char data without copying and withoug keeping source std::string object ? (I want to use moving semantics but between different types)
你不能安全地做到这一点.
对于具体的实现,在某些情况下,您可以使用别名来修改字符串中的私有成员变量来欺骗字符串,认为它不再拥有缓冲区.但是即使你愿意尝试这样做也不会总是奏效.例如.考虑小字符串优化,其中字符串没有指向一些保存数据的外部缓冲区的指针,数据在字符串对象本身内.
如果您想避免复制,可以考虑将界面更改为scheduledWrite.一种可能性是：
template<typename Container>
void scheduledWrite(Container data)
{
// requires data[i],data.size(),and &data[n] == &data[0] + n for n [0,size)
…
}
// move resources from object owned by a unique_ptr
WriteId write( std::unique_ptr< std::vector<char> > vecToWrite)
{
scheduleWrite(std::move(*vecToWrite));
}
WriteId write( std::unique_ptr< std::string > strToWrite)
{
scheduleWrite(std::move(*strToWrite));
}
// move resources from object passed by value (callers also have to take care to avoid copies)
WriteId write(std::string strToWrite)
{
scheduleWrite(std::move(strToWrite));
}
// assume ownership of raw pointer
// requires data to have been allocated with new char[]
WriteId write(char const *data,size_t size) // you could also accept an allocator or deallocation function and make ptr_adapter deal with it
{
struct ptr_adapter {
std::unique_ptr<char const []> ptr;
size_t m_size;
char const &operator[] (size_t i) { return ptr[i]; }
size_t size() { return m_size; }
};
scheduleWrite(ptr_adapter{data,size});
}
