std::ostringstream printing the address of the c-string instead of its content

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The expressionstd::ostringstream() creates a temporary, and operator<< which takes const char* as argument is a free function, but this free function cannot be called on a temporary, as the type of the first parameter of the function is std::ostream& which cannot be bound to temporary object.

Having said that, <<std::ostringstream() << "some data" resolves to a call to a member function which is overloaded for void* which prints the address. Note that a member function can be invoked on the temporary.

In order to call the free function, you need to convert temporary (which is rvalue) into a lvalue, and here is one trick that you can do:

 std::cout << "Inline        : "
            << dynamic_cast<std::ostringstream&>(
                 std::ostringstream().flush() << "some data"
               ).str()
            << "\n";

That is, std::ostringstream().flush() returns std::ostream& which means, now the free function can called, passing the returned reference as first argument.

Also, you don’t need to use dynamic_cast here (which is slow, as it is done at runtime), for the type of the object is pretty much known, and so you can use static_cast (which is fast as it is done at compile-time):

 std::cout << "Inline        : "
            << static_cast<std::ostringstream&>(
                 std::ostringstream().flush() << "some data"
               ).str()
            << "\n";

which should work just fine.

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