c++ - Turning temporary stringstream to c_str() in single statement

Question

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_cast<std::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<std::ostringstream&>(static_cast<std::ostringstream&>(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<std::ostringstream&>(static_cast<std::ostringstream&>(std::ostringstream()) << x).str().c_str())

// ...

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

Would this function as expected?

Answer 1

You cannot cast the temporary stream to std::ostringstream&. It is ill-formed (the compiler must tell you that it is wrong). The following can do it, though:

f(static_cast<std::ostringstream&>(
  std::ostringstream().seekp(0) << "Value: " << 5).str().c_str());

That of course is ugly. But it shows how it can work. seekp is a member function returning a std::ostream&. Would probably better to write this generally

template<typename T>
struct lval { T t; T &getlval() { return t; } };

f(static_cast<std::ostringstream&>(
  lval<std::ostringstream>().getlval() << "Value: " << 5).str().c_str());

The reason that without anything it takes the void*, is because that operator<< is a member-function. The operator<< that takes a char const* is not.