C ++模板运算符编译错误

问题描述

我试图让一个类似于 c >：

  template< typename cT，typename Traits> 
 std :: basic_ostream< cT，Traits>& std :: baisic_ostream< cT，Traits> :: operator<< （
 std :: basic_ostream< cT，Traits>&（* manip）（std :: basic_ostream< cT，Traits>&）） 
  

这样，编译器可以推导出 std :: endl 。

然而，所有这一切是完全不相关的，因为你想要做的是完全不同的！您应该创建一个合适的流缓冲区，并使用这个自定义流缓冲区合理构造的 std :: ostream 。下面是一个完整的例子，如何正确地（我已经发布了，但只有几十次...）：

  #include< streambuf> 
 
 struct teebuf 
：std :: streambuf 
 {
 std :: streambuf * sb1_; 
 std :: streambuf * sb2_; 
 
 teebuf（std :: streambuf * sb1，std :: streambuf * sb2）
：sb1_（sb1），sb2_（sb2）{
} 
 （int c）{
 typedef std :: streambuf :: traits_type traits; 
 bool rc（true）; 
 if（！traits :: eq_int_type（traits :: eof（），c））{
 traits :: eq_int_type（this-> sb1 _-> sputc（c），traits :: eof ））
&&& （rc = false）; 
 traits :: eq_int_type（this-> sb2 _-> sputc（c），traits :: eof（））
&& （rc = false）; 
} 
 return rc？ traits :: not_eof（c）：traits :: eof（）; 
} 
 int sync（）{
 bool rc（true）; 
 this-> sb1 _-> pubsync（）！=  -  1 || （rc = false）; 
 this-> sb2 _-> pubsync（）！= -1 || （rc = false）; 
 return rc？ 0：-1; 
} 
}; 
 
 #include< fstream> 
 #include< iostream> 
 
 int main（）
 {
 std :: ofstream fout（tee.txt）; 
 teebuf sbuf（fout.rdbuf（），std :: cout.rdbuf（））; 
 std :: ostream out（& sbuf）; 
 out<< hello，world！\\\
; 
} 
  

I'm trying to make a C++ class resembling std::ostream, that will take its input and write to two std::ostreams given in the constructor. Here it is together with appropriate operator<< template:

struct SplitStream
{
    SplitStream(std::ostream & a_, std::ostream & b_) : a(a_), b(b_) {}
    std::ostream & a, & b;
};


template<class T>
const SplitStream & operator << (const SplitStream & sp, const T & x)
{
    sp.a << x;
    sp.b << x;
    return sp;
}

Several lines below that code, I try to use this class:

void foo(SplitStream & out)
{
    double some_double = 1.23;
    out << "bar" << some_double << std::endl;
}

And I get this rather enigmatic error:

... error C2678: binary '<<' : no operator found which takes a left-hand operand of type 'const SplitStream' (or there is no acceptable conversion) ...

What am I doing wrong? I tried to define operator<< without consts, and it didn't compile either.

解决方案

The immediate problem is that std::endl isn't an object but a function template declared something like this:

template <typename cT, typename Traits>
std::basic_ostream<cT, Traits>& endl(std::basic_ostream<cT, Traits>&);

To use a function pointer like this, the template arguments need to be deduced. To this end, the class std::basic_ostream<cT, Traits> declared suitable overloads for operator<<():

template <typename cT, typename Traits>
std::basic_ostream<cT, Traits>& std::baisic_ostream<cT, Traits>::operator<< (
    std::basic_ostream<cT, Traits>& (*manip)(std::basic_ostream<cT, Traits>&));

This way, the compiler can deduce the correct instantiation when the std::endl function is referenced.

However, all that is utterly irrelevant because what you are trying to do is better done entirely differently! You should create a suitable stream buffer and use a reasonably constructed std::ostream with this custom stream buffer. Below is a complete example how to do it properly (I had posted it before but only a couple dozens times...):

#include <streambuf>

struct teebuf
    : std::streambuf
{
    std::streambuf* sb1_;
    std::streambuf* sb2_;

    teebuf(std::streambuf* sb1, std::streambuf* sb2)
        : sb1_(sb1), sb2_(sb2) {
    }
    int overflow(int c) {
        typedef std::streambuf::traits_type traits;
        bool rc(true);
        if (!traits::eq_int_type(traits::eof(), c)) {
            traits::eq_int_type(this->sb1_->sputc(c), traits::eof())
                && (rc = false);
            traits::eq_int_type(this->sb2_->sputc(c), traits::eof())
                && (rc = false);
        }
        return rc? traits::not_eof(c): traits::eof();
    }
    int sync() {
        bool rc(true);
        this->sb1_->pubsync() != -1 || (rc = false);
        this->sb2_->pubsync() != -1 || (rc = false);
        return rc? 0: -1;
    }
};

#include <fstream>
#include <iostream>

int main()
{
    std::ofstream fout("tee.txt");
    teebuf        sbuf(fout.rdbuf(), std::cout.rdbuf());
    std::ostream  out(&sbuf);
    out << "hello, world!\n";
}

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