使用ostringstream语法将字符串加倍

Question

I'd like to understand why writing this

static_cast<std::ostringstream*>( &(std::ostringstream() << speed.x) )->str();

makes a string, but not this

(std::ostringstream() << speed.x).str();?

in fact the latter doesn't even compile...

I find this static_cast<foo*>&foo to be quite weird.

can you give me good examples in which case it's good practice to do so?

Answer 1

The the expression std::ostringstream() << speed.x actually invokes the operator<<(double) on the underlying base class std::ostream interface.

The return type of std::ostream::operator<<(double) is std::ostream& which means you're trying to invoke the member function std::ostream::str() which of course does not exist. That method is on the derived class.

This is why the static_cast is necessary in this use case.

You could also write:

static_cast<std::ostringstream&>(std::ostringstream() << speed.x).str();

or since c++11

std::to_string(speed.x);

or in previous versions, you could write your own, less cryptic function which will do the same thing in a more maintainable way with no overhead.

std::string to_string(double x)
{
    std::ostringstream ss;
    ss << x;
    return ss.str();
}