one else for two if possible?

QT-static-prgm

Hi,

my question is more a general c++ question then Qt, but i hope you can help me with that, too.

Example 1:
first condition: the index is not greater then the container
2nd condition: the value in the container at the index is set
if one of those is not given, the else should be executed.

So it need to be like this:

if (segmentIterator->textureIndex < m_materials->size() && m_materials->at(segmentIterator->textureIndex).transparent)
	m_drawList.push_back(new_info);
else
	m_drawList.push_front(new_info);

problem: if the first condition is false, i cannot even test the 2nd condition, so it need to be like this:

if (segmentIterator->textureIndex < m_materials->size())
        if(m_materials->at(segmentIterator->textureIndex).transparent)
        	m_drawList.push_back(new_info);
else
	m_drawList.push_front(new_info);

problem: dangling else. the compiler will read it like this:

if (segmentIterator->textureIndex < m_materials->size())
{
	if(m_materials->at(segmentIterator->textureIndex).transparent)
		m_drawList.push_back(new_info);
	else
		m_drawList.push_front(new_info);
}

Any solutions for that?? In theory an good compiler should optimize the code and if the first condition of a && expression is false, the 2nd shouldn't be even touched. But i'm not sure if that works. Any ideas about that??

VRonin

usa a variable...

bool check = segmentIterator->textureIndex < m_materials->size();
if(check) check =m_materials->at(segmentIterator->textureIndex).transparent;
if(check) 
m_drawList.push_back(new_info);
else
m_drawList.push_front(new_info);

jsulm

@QT-static-prgm said in one else for two if possible?:

problem: if the first condition is false, i cannot even test the 2nd condition, so it need to be like this:

What is the problem? Why do you want to test the second condition if the first is false?
As far as I can see the second condition does not have side effects, so it should be fine.

QT-static-prgm

@jsulm i want to test the 2nd condition, but to prevent errors, i check the index to be not out of the range.

btw for me works

if (segmentIterator->textureIndex < m_materials->size() && m_materials->at(segmentIterator->textureIndex).transparent)
	m_drawList.push_back(new_info);
else
	m_drawList.push_front(new_info);

But i'm not sure if it is compiler specific.

Chris Kawa

Your code is ok.
Order of evaluation for && operator is specified and it is left to right. && also uses short circuit evaluation, so the right side is evaluated if and only if the left side is evaluated to true.

QT-static-prgm

@Chris-Kawa all right, thank you. I just wasn't sure if it was only theory in my university lecture, or if it is implemented in all C++ compiler

VRonin

@Chris-Kawa said in one else for two if possible?:

also uses short circuit evaluation

Disclaimer: I'm not 100% sure here

I don't think this holds up in case of an overloaded operator&& so if transparent returns a type that has on overloaded operator&& that interacts with bool, the right part of the logical AND will be executed regardless of what the left part returns

Chris Kawa

@VRonin You're 100% right. That's a cool nitpick. I like it :D

In this case though I doubt it matters. As you said transparent would have to be a custom class and there would have to be a Something operator&&(bool a, const SomeClass& b) defined in the scope. That's not very common.

QT-static-prgm

Ähm yes, transparent is a boolean value of a struct.