c++ simple stream manipulation with ostream and istream? - c++

Question

I have been looking for a solution but couldn't find what I need/want.
All I want to do is pass a stream intended for std::cout to a function, which manipulates it. What I have used so far is a template function:
template<typename T>
void printUpdate(T a){
std::cout << "blabla" << a << std::flush;
}
int main( int argc, char** argv ){
std::stringstream str;
str << " hello " << 1 + 4 << " goodbye";
printUpdate<>( str.str() );
return 0;
}
What I would prefer is something like:
printUpdate << " hello " << 1 + 4 << " goodbye";
or
std::cout << printUpdate << " hello " << 1 + 4 << " goodbye";
I was trying to do:
void printUpdate(std::istream& a){
std::cout << "blabla" << a << std::flush;
}
but that gave me:
error: invalid operands of types ‘void(std::istream&) {aka void(std::basic_istream<char>&)}’ and ‘const char [5]’ to binary ‘operator<<’

Answer 1

You can't output data to an input stream, just not a good thing to do.
Change:
void printUpdate(std::istream& a){
std::cout << "blabla" << a << std::flush;
}
To:
void printUpdate(std::ostream& a){
std::cout << "blabla" << a << std::flush;
}
Note the stream type change.
Edit 1:
Also, you can't output a stream to another stream, at least std::cout.
The return value of << a is a type ostream.
The cout stream doesn't like being fed another stream.
Change to:
void printUpdate(std::ostream& a)
{
static const std::string text = "blabla";
std::cout << text << std::flush;
a << text << std::flush;
}
Edit 2:
You need to pass a stream to a function requiring a stream.
You can't pass a string to a function requiring a stream.
Try this:
void printUpdate(std::ostream& out, const std::string& text)
{
std::cout << text << std::flush;
out << text << std::flush;
}
int main(void)
{
std::ofstream my_file("test.txt");
printUpdate(my_file, "Apples fall from trees.\n");
return 0;
}
Chaining Output Streams
If you want to chain things to the output stream, like results from functions, the functions either have to return a printable (streamable object) or the same output stream.
Example:
std::ostream& Fred(std::ostream& out, const std::string text)
{
out << "--Fred-- " << text;
return out;
}
int main(void)
{
std::cout << "Hello " << Fred("World!\n");
return 0;
}