Turning temporary stringstream to c_str() in single statement
- by AshleysBrain
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?
