C++ Using stringstream after << as parameter

问题:

Is it possible to write a method that takes a stringstream and have it look something like this,

void method(string str)
void printStringStream( StringStream& ss)
{
    method(ss.str());
}

And can be called like this

stringstream var;
printStringStream( var << "Text" << intVar << "More text"<<floatvar);

I looked up the << operator and it looks like it returns a ostream& object but I'm probably reading this wrong or just not implementing it right.

Really all I want is a clean way to concatenate stuff together as a string and pass it to a function. The cleanest thing I could find was a stringstream object but that still leaves much to be desired.

Notes:

I can't use much of c++11 answers because I'm running on Visual Studio 2010 (against my will, but still)

I have access to Boost so go nuts with that.

I wouldn't be against a custom method as long as it cleans up this mess.

Edit:

With @Mooing Duck's answer mixed with @PiotrNycz syntax I achieved my goal of written code like this,

try{

    //code

}catch(exception e)
{   
    printStringStream( stringstream() << "An exception has occurred.\n"
                            <<"    Error: " << e.message 
                            <<"\n If this persists please contact "<< contactInfo
                            <<"\n Sorry for the inconvenience");
}

This is as clean and readable as I could have hoped for.

Hopefully this helps others clean up writing messages.

回答1:

Ah, took me a minute. Since operator<< is a free function overloaded for all ostream types, it doesn't return a std::stringstream, it returns a std::ostream like you say.

void printStringStream(std::ostream& ss)

Now clearly, general ostreams don't have a .str() member, but they do have a magic way to copy one entire stream to another:

std::cout << ss.rdbuf();

Here's a link to the full code showing that it compiles and runs fine http://ideone.com/DgL5V

EDIT

If you really need a string in the function, I can think of a few solutions:

First, do the streaming seperately:

stringstream var;
var << "Text" << intVar << "More text"<<floatvar;
printStringStream(var);

Second: copy the stream to a string (possible performance issue)

void printStringStream( ostream& t)
{
    std::stringstream ss;
    ss << t.rdbuf();
    method(ss.str());
}

Third: make the other function take a stream too

回答2:

Make your wrapper over std::stringstream. In this new class you can define whatever operator << you need:

class SSB {
public:
   operator std::stringstream& () { return ss; }

   template <class T>
   SSB& operator << (const T& v) { ss << v; return *this; }
   template <class T>
   SSB& operator << (const T* v) { ss << v; return *this; }
   SSB& operator << (std::ostream& (*v)(std::ostream&)) { ss << v; return *this; }
   // Be aware - I am not sure I cover all <<'s       
private:
   std::stringstream ss;
};

void print(std::stringstream& ss)
{
    std::cout << ss.str() << std::endl;
}

int main() {
  SSB ssb;
  print (ssb << "Hello" << " world in " << 2012 << std::endl);
  print (SSB() << "Hello" << " world in " << 2012 << std::endl);
}

回答3:

For ease of writing objects that can be inserted into a stream, all these classes overload operator<< on ostream&. (Operator overloading can be used by subclasses, if no closer match exists.) These operator<< overloads all return ostream&.

What you can do is make the function take an ostream& and dynamic_cast<> it to stringstream&. If the wrong type is passed in, bad_cast is thrown.

void printStringStream(ostream& os) {
    stringstream &ss = dynamic_cast<stringstream&>(os);
    cout << ss.str();
}

Note: static_cast<> can be used, it will be faster, but not so bug proof in the case you passed something that is not a stringstream.

回答4:

Since you know you've got a stringstream, just cast the return value:

stringstream var;
printStringStream(static_cast<stringstream&>(var << whatever));

回答5:

Just to add to the mix: Personally, I would create a stream which calls whatever function I need to call upon destruction:

#include <sstream>
#include <iostream>

void someFunction(std::string const& value)
{
    std::cout << "someFunction(" << value << ")\n";
}

void method(std::string const& value)
{
    std::cout << "method(" << value << ")\n";
}

class FunctionStream
    : private virtual std::stringbuf
    , public std::ostream
{
public:
    FunctionStream()
        : std::ostream(this)
        , d_function(&method)
    {
    }
    FunctionStream(void (*function)(std::string const&))
    : std::ostream(this)
    , d_function(function)
    {
    }
    ~FunctionStream()
    {
        this->d_function(this->str());
    }
private:
    void (*d_function)(std::string const&);
};

int main(int ac, char* av[])
{
    FunctionStream() << "Hello, world: " << ac;
    FunctionStream(&someFunction) << "Goodbye, world: " << ac;
}

It is worth noting that the first object sent to the temporary has to be of a specific set of types, namely one of those, the class std::ostream knows about: Normally, the shift operator takes an std::ostream& as first argument but a temporary cannot be bound to this type. However, there are a number of member operators which, being a member, don't need to bind to a reference! If you want to use a user defined type first, you need to extract a reference temporary which can be done by using one of the member input operators.