What is the difference between... (casting question)

**yaya** · 10-01-2009

I think I'm having a brain f*** at the moment. Why does this work:

Code:

#include <fstream>
#include <string>

int main()
{
	char temp;
	std::string str( "this is a string" );
	std::ofstream out( "test.txt", ios::trunc );

	temp = (char)str.size();
	out.write( &temp, 1 );

	out.close();
	
	return 0;
};

...yet this doesn't:

Code:

#include <fstream>
#include <string>

int main()
{
	std::string str( "this is a string" );
	std::ofstream out( "test.txt", ios::trunc );

	out.write( &((char)str.size()), 1 ); // ERROR HERE!!

	out.close();
	
	return 0;
};

The error I get is:

error C2102: '&' requires l-value

But shouldn't they be the same thing?

**quzah** · 10-01-2009

Because 'temp' is an actual variable, whereas the value returned by str.size() is just a number (value). You can't get the address of a value.

That's like saying:

Code:

out.write( &5, 1 );

Quzah.

**yaya** · 10-01-2009

Ah, I see. I should've known that. Thanks.

**King Mir** · 10-01-2009

That's not quite right.

Functions generally give their return value on the stack. After that value is processed, the stack is reused by the next function call. In this case the only processing is taking the address, which will wind up as a pointer to a portion of the stack.

**quzah** · 10-01-2009

The function doesn't return an address. It returns a value. It has nothing to do with the stack. It has to do with you not being able to get the address of an actual value.

Quzah.

**King Mir** · 10-01-2009

You're right. I was assuming it compiled, in which case the behavior would be as I described, for any real implementation.

But yeah it doesn't compile, so it's just that you can't take the address of an rvalue.

**CornedBee** · 10-02-2009

And even if it compiled, this would still be wrong:

Functions generally give their return value on the stack.

I don't know that many architectures, but those I do know (x86, x86-64, Alpha, PowerPC) all return sufficiently small values in a register.

**EVOEx** · 10-02-2009

Originally Posted by CornedBee

And even if it compiled, this would still be wrong:

I don't know that many architectures, but those I do know (x86, x86-64, Alpha, PowerPC) all return sufficiently small values in a register.

Seconded that. Specifically, x86 and amd64 store return values in the register [e/r]ax. So it has nothing to do with a stack. Hence it doesn't have an address, as registers don't have addresses as they're not in RAM.
(That is, for values that fit in the register. For bigger items, such as struct, the items are stored on the stack and [e/r]ax holds its address. At least on my x86.)

And to the original poster:
Don't forget to open your file as binary.

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