Error in release build only, copying structure from std::vector

Question

I'm having a weird problem trying to get my release build working in Xcode 4.2 using llvm. I've turned off all optimisation settings for the release scheme, and as far as I can tell the release build matches all the settings of the debug build. Regardless of this, the following problem occurs when working with some structures from Box2D, a physics library - but I am unsure if the problem has anything todo specifically with that.

b2CircleShape* circleShape = new b2CircleShape();

circleShape->m_p.Set(0,0);
circleShape->m_radius = m_radius;

b2FixtureDef fixture;
fixture.shape = circleShape;
fixture.density = m_density;

m_fixtureDefs.push_back(fixture); // std::vector

b2FixtureDef fix2 = fixture;
b2FixtureDef fix3 = m_fixtureDefs[0] // EXC_BAD_ACCESS

When I remove all instances of access to m_fixtures, no problems occur. When I run in the development scheme no errors occur. I am really, really confused, if someone could point me in the right direction for errors to look for it would be much appreciated

EDIT:

More interesting stuff

for (vector<b2FixtureDef>::iterator i = m_fixtureDefs.begin() ; i != m_fixtureDefs.end(); ++i)
{

}

This appears to loop forever, making me very confused. It seems like the structure m_fixturesDef has some kind of problem, but I have no idea why whatever weird corruption is going on is only occurring in this particular variable.

Answer 1

By default POD objects are not initializes in C++ so their initial value (unless explicitly initialized) are inherently random.

When you build in debug mode the compiler usually inserts extra initialization code to zero out values. Thus you can easily see different behaviors between debug and release builds.

A quick way to find this kind of problem is to check your compiler warnings; see if you are using a variable before it has been initialization (you may need to turn on the warnings) or something similar.

Note: You can fix a lot of serious problems by making sure your code compiles with zero warnings with the warning level as high as reasonable (usually a step above default). (a warning is really a logical error in your code).