“错误C3867：非标准语法； 使用虚拟流操纵器时，使用“＆”创建指向成员的指针”

Question

In my code in Visual Studio 2015 i have interface:

struct IStr
{
    virtual std::ostream& beginMessage() = 0;
    virtual std::ostream& endMessage(std::ostream&) = 0;
};

And i have a class that implements this interface, like this:

#include <sstream>

struct MyStr : public IStr
{
    std::stringstream m_stream;
    std::ostream& beginMessage() override { return m_stream; }
    std::ostream& endMessage(std::ostream& ss) override { return std::endl(ss); }
};

However, i am getting an error when trying to compile simple code:

IStr * pStr = new MyStr();
pStr->beginMessage() << "Hello Wordl!" << pStr->endMessage;

With message:

Error   C3867   
'IStr::endMessage': non-standard syntax; use '&' to create a pointer to member

I really like syntax i am trying to use. But is this possible? Maybe the problem is that my manipulator is virtual, or non-static?

Answer 1

Use NVI, and have endMessage() return a (stateful) manipulator that calls the virtual function on the stream when streamed.

struct IStr
{
    // other stuff

    private: 
    virtual std::ostream& doBeginMessage() = 0;
    virtual std::ostream& doEndMessage(std::ostream&) = 0;

    struct EndManip{
        IStr* istr;
    };
    friend std::ostream& operator<<(std::ostream& ss, EndManip em){
        return em.istr->doEndMessage(ss);
    }
    public:
    EndManip endMessage() { return {this}; }
    std::ostream& beginMessage() { return doBeginMessage(); }
};

With this you'd do pStr->beginMessage() << "Hello World!" << pStr->endMessage(); pStr->beginMessage() << "Hello World!" << pStr->endMessage(); .

Answer 2

You have written:

IStr * pStr = new MyStr();
pStr->beginMessage() << "Hello Wordl!" << pStr->endMessage;

while you should have written something similar to:

IStr * pStr = new MyStr();
pStr->beginMessage() << "Hello Wordl!" << pStr->endMessage(*pStream);

The difference here is in the (), which results in doing the function call instead of trying to pass a pointer to the function (which is most likely not your intent).