Consider the following function:

void f(const char* str);

Suppose I want to generate a string using stringstream and pass it to this function. If I want to do it in one statement, I might try:

f((std::ostringstream() << "Value: " << 5).str().c_str()); // error

This gives an error:

'str()' is not a member of 'basic_ostream'.

OK, so operator<< is returning ostream instead of ostringstream - how about casting it back to an ostringstream?

1. Is this cast safe?

   f(static_caststd::ostringstream&(std::ostringstream() << "Value: " << 5).str().c_str()); // incorrect output

Now with this, it turns out for the operator<<("Value: ") call, it's actually calling ostream's operator<<(void*) and printing a hex address. This is wrong, I want the text.

2. Why does operator<< on the temporary std::ostringstream() call the ostream operator? Surely the temporary has a type of 'ostringstream' not 'ostream'?

I can cast the temporary to force the correct operator call too!

f(static_cast(static_cast(std::ostringstream()) << "Value: " << 5).str().c_str());

This appears to work and passes "Value: 5" to f().

3. Am I relying on undefined behavior now? The casts look unusual.

I'm aware the best alternative is something like this:

std::ostringstream ss;
ss << "Value: " << 5;
f(ss.str().c_str());

...but I'm interested in the behavior of doing it in one line. Suppose someone wanted to make a (dubious) macro:

#define make_temporary_cstr(x) (static_cast(static_cast(std::ostringstream()) << x).str().c_str())

// ...

f(make_temporary_cstr("Value: " << 5));

Would this function as expected?