問題描述
我需要通過一個(gè)流橋接兩個(gè)庫.
I need to bridge two libraries over a stream.
QDataStream which is a stream from Qt
和其他庫中的一些函數(shù)，看起來像這樣
and some function from another libraries that looks like this
void read_something(istream& i);
我無法控制 QDataStream 的創(chuàng)建方式，也不允許更改 read_somthing 函數(shù)的接口.
I have no control over how the QDataStream is created and I'm not allowed to change the interface of read_somthing function.
我首先想到的就是寫一個(gè)繼承istream并包裝QDataStream的類.以前有人這樣做過嗎?
The first thing I can think of is write a class that inherits istream and wraps QDataStream. Have anybody done that before?
如果我認(rèn)為的方法不正確，我想知道實(shí)現(xiàn)這一目標(biāo)的最佳方法是什么.
If what I thought wasn't the proper way, I wonder what is the best way to achieve that.
推薦答案
你應(yīng)該寫一個(gè)流緩沖，它使用 QDataStream readBytes 和 writeBytes 來實(shí)現(xiàn)它的功能.然后使用 rdbuf 將 streambuf 注冊(cè)到 istream(您也可以編寫一個(gè) istream 后代，在初始化時(shí)執(zhí)行此操作).
What you should do is write a streambuf which uses the QDataStream readBytes and writeBytes to implement its functions. Then register the streambuf into a istream with rdbuf (you can also write an istream descendant which does this when initialized).
Boost 包含一個(gè)旨在促進(jìn) streambuf 寫入的庫.使用它可能比理解 streambuf 接口更簡單(我個(gè)人從未使用過它，但我已經(jīng)編寫了多個(gè) streambuf；我會(huì)看看我是否有可以發(fā)布的示例).
Boost contains a library aiming at facilitating the writing of streambuf. It could be simpler to use it than understanding the streambuf interface (personally I never have used it but I've written multiple streambuf; I'll see if I've a example that I can post).
這里有一些東西(用法語評(píng)論——它來自fr.comp.lang.c++的法語常見問題解答——，我沒有時(shí)間翻譯，認(rèn)為留下它們比刪除它們更好)它將 FILE* 調(diào)用包裝成一個(gè)流緩沖.這也是使用私有繼承的一個(gè)示例:確保在基類之前初始化可能是成員的內(nèi)容.在 IOStream 的情況下，基類也可以接收一個(gè) NULL 指針，然后成員 init() 用于設(shè)置流緩沖區(qū).
here is something (commented in French -- it comes from the french FAQ of fr.comp.lang.c++ --, I have no time for translation and think it is better to leave them than to remove them) which wraps FILE* call into a streambuf. This also is a show case of a use of private inheritance: ensuring that what could be a member is initialized before a base class. In the case of IOStream, the base class could as well receive a NULL pointer and then the member init() used to set the streambuf.
#include <stdio.h>
#include <assert.h>
#include <iostream>
#include <streambuf>
// streambuf minimal encapsulant un FILE*
// - utilise les tampons de FILE donc n'a pas de tampon interne en
// sortie et a un tampon interne de taille 1 en entree car l'interface
// de streambuf ne permet pas de faire moins;
// - ne permet pas la mise en place d'un tampon
// - une version plus complete devrait permettre d'acceder aux
// informations d'erreur plus precises de FILE* et interfacer aussi
// les autres possibilites de FILE* (entre autres synchroniser les
// sungetc/sputbackc avec la possibilite correspondante de FILE*)
class FILEbuf: public std::streambuf
{
public:
explicit FILEbuf(FILE* cstream);
// cstream doit etre non NULL.
protected:
std::streambuf* setbuf(char_type* s, std::streamsize n);
int_type overflow(int_type c);
int sync();
int_type underflow();
private:
FILE* cstream_;
char inputBuffer_[1];
};
FILEbuf::FILEbuf(FILE* cstream)
: cstream_(cstream)
{
// le constructeur de streambuf equivaut a
// setp(NULL, NULL);
// setg(NULL, NULL, NULL);
assert(cstream != NULL);
}
std::streambuf* FILEbuf::setbuf(char_type* s, std::streamsize n)
{
// ne fait rien, ce qui est autorise. Une version plus complete
// devrait vraissemblablement utiliser setvbuf
return NULL;
}
FILEbuf::int_type FILEbuf::overflow(int_type c)
{
if (traits_type::eq_int_type(c, traits_type::eof())) {
// la norme ne le demande pas exactement, mais si on nous passe eof
// la coutume est de faire la meme chose que sync()
return (sync() == 0
? traits_type::not_eof(c)
: traits_type::eof());
} else {
return ((fputc(c, cstream_) != EOF)
? traits_type::not_eof(c)
: traits_type::eof());
}
}
int FILEbuf::sync()
{
return (fflush(cstream_) == 0
? 0
: -1);
}
FILEbuf::int_type FILEbuf::underflow()
{
// Assurance contre des implementations pas strictement conformes a la
// norme qui guaranti que le test est vrai. Cette guarantie n'existait
// pas dans les IOStream classiques.
if (gptr() == NULL || gptr() >= egptr()) {
int gotted = fgetc(cstream_);
if (gotted == EOF) {
return traits_type::eof();
} else {
*inputBuffer_ = gotted;
setg(inputBuffer_, inputBuffer_, inputBuffer_+1);
return traits_type::to_int_type(*inputBuffer_);
}
} else {
return traits_type::to_int_type(*inputBuffer_);
}
}
// ostream minimal facilitant l'utilisation d'un FILEbuf
// herite de maniere privee de FILEbuf, ce qui permet de s'assurer
// qu'il est bien initialise avant std::ostream
class oFILEstream: private FILEbuf, public std::ostream
{
public:
explicit oFILEstream(FILE* cstream);
};
oFILEstream::oFILEstream(FILE* cstream)
: FILEbuf(cstream), std::ostream(this)
{
}
// istream minimal facilitant l'utilisation d'un FILEbuf
// herite de maniere privee de FILEbuf, ce qui permet de s'assurer
// qu'il est bien initialise avant std::istream
class iFILEstream: private FILEbuf, public std::istream
{
public:
explicit iFILEstream(FILE* cstream);
};
iFILEstream::iFILEstream(FILE* cstream)
: FILEbuf(cstream), std::istream(this)
{
}
// petit programme de test
#include <assert.h>
int main(int argc, char* argv[])
{
FILE* ocstream = fopen("result", "w");
assert (ocstream != NULL);
oFILEstream ocppstream(ocstream);
ocppstream << "Du texte";
fprintf(ocstream, " melange");
fclose(ocstream);
FILE* icstream = fopen("result", "r");
assert (icstream != NULL);
iFILEstream icppstream(icstream);
std::string word1;
std::string word2;
icppstream >> word1;
icppstream >> word2;
char buf[1024];
fgets(buf, 1024, icstream);
std::cout << "Got :" << word1 << ':' << word2 << ':' << buf << '
';
}
這篇關(guān)于繼承 std::istream 或等價(jià)物的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！