没有合适的默认构造函数可用于初始化 ostream 指针

no appropriate default constructor available initialising ostream pointer

本文关键字：用于 初始化 ostream 指针 构造函数 默认      更新时间：2023-10-16

我正在尝试创建一个这样的日志记录类：

#include <iostream>
#include <fstream>
class logstream : public std::ostream
{
public:
  logstream() : os(&std::cout), file_(false) {}  // line 7
  logstream(const char* filename) : os(new std::ofstream("file.txt")), file_(true) {
    // open file - raise exception if cannot open
  }
  ~logstream() {
     // check if file and if so close
    if(file_) {
      os->close();
      delete os;
    }      
  }
private:
  std::ostream* os;
  bool file_;
};

但是我在第 7 行收到此错误：

error C2512: 'std::basic_ostream<_Elem,_Traits>' : no appropriate default constructor available
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]

第 9 行也有同样的错误：

error C2512: 'std::basic_ostream<_Elem,_Traits>' : no appropriate default constructor available
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]

我还需要在 os->close 行上修复此错误：

error C2039: 'close' : is not a member of 'std::basic_ostream<_Elem,_Traits>'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]

任何想法如何解决这些编译错误？

更新

所以我可以这样做：

class logstream 
{
public:
  logstream() : os(&std::cout), file_(false) {}
  logstream(const char* filename) : os(new std::ofstream("file.txt")), file_(true) {
    // open file - raise exception if cannot open
  }
  ~logstream() {
     // check if file and if so close
    if(file_) {
      static_cast<std::ofstream*>(os)->close();
      delete os;
    }      
  }
private:
  std::ostream* os;
  bool file_;
};

使用内部 ostream 对象。 或者我可以从 ostream 继承并删除内部 ostream ptr。 但是我将如何为继承版本编写构造函数呢？

我在这个版本上收到错误：

class logstream2 : public std::ostream
{
public:
  logstream2() : std::ostream(std::cout) {}
  logstream2(const char* filename) : std::ostream(std::ofstream("file.txt")) {
    // open file - raise exception if cannot open - check state of stream
  }
  ~logstream2() {
    // not sure if I need this     
  }
};

错误：

error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private member declared in class 'std::basic_ios<_Elem,_Traits>'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        c:program files (x86)microsoft visual studio 9.0vcincludeios(151) : see declaration of 'std::basic_ios<_Elem,_Traits>::basic_ios'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        This diagnostic occurred in the compiler generated function 'std::basic_ostream<_Elem,_Traits>::basic_ostream(const std::basic_ostream<_Elem,_Traits> &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]

这就是我最终得到的。

#include <iostream>
#include <fstream>
class logstream
{
public:
  logstream() : os(&std::cout), file_(false) {}
  logstream(const char* filename) : os(new std::ofstream(filename)), file_(true) {
    // open file - raise exception if cannot open - TODO
  }
  ~logstream() {
     // check if file and if so close
    if(file_) {
      static_cast<std::ofstream*>(os)->close();
      delete os;
    }      
  }
  template<typename T>
  std::ostream& operator<<(const T& o) {
    return *os << o;
  }
private:
  std::ostream* os;
  bool file_;
};

int main () {
  logstream log("logtest.txt");
  log << "Hi Guys!n";
  log << "number: " << 3 << std::endl;
  return 0;
}

这个呢？

#include <iostream>
#include <ostream>
#include <fstream>
#include <string>
#include <memory>
class logstream
{
public:
    logstream() : os(new std::ostream(std::cout.rdbuf())), file_(false) {}  // line 7
    logstream(const std::string &filename) :
        os(new std::ofstream(filename)), file_(true)
    {
        // open file - raise exception if cannot open
    }
    std::ostream &operator<<(const std::string &s)
    {
        *os << s;
    }
    ~logstream()
    {
        // check if file and if so close
        if (file_)
        {
            dynamic_cast<std::ofstream*>(os.get())->close();
        }
    }
private:
    std::unique_ptr<std::ostream> os;
    bool file_;
};
int main()
{
    logstream l("test.txt");
    l << "some test"; // write to test.txt
    logstream another;
    another << "another test"; //write to std::cout by default
}

您需要在初始值设定项列表中调用从中继承的类的构造函数 std::ostream 。

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